It was the early evening of 5 February 2013, and seated among grave-looking men in suits, a woman named Ada Colau was about to give evidence to a Spanish parliamentary hearing. “Before saying anything,” she began, “I’d just like to make one thing clear. I am not an important person. I have never held office or been the president of anything … The only reason I am here is that I am a momentarily visible face of a citizens’ movement.”

Colau was there to discuss the housing crisis that had devastated Spain. Since the financial crisis, 400,000 homes had been foreclosed and a further 3.4m properties lay empty. In response, Colau had helped to set up a grassroots organisation, the Platform for Mortgage Victims (PAH), which championed the rights of citizens unable to pay their mortgages or threatened with eviction. Founded in 2009, the PAH quickly became a model for other activists, and a nationwide network of leaderless local groups emerged. Soon, people across Spain were joining together to campaign against mortgage lenders, occupy banks and physically block bailiffs from carrying out evictions.

Ten minutes into Colau’s 40-minute testimony she broke from the script. Her voice cracking with emotion, she turned her attention to the previous speaker, Javier Rodriguez Pellitero, the deputy general secretary of the Spanish Banking Association: “This man is a criminal, and should be treated as such. He is not an expert. The representatives of financial institutions have caused this problem; they are the same people who have caused the problem that has ruined the entire economy of this country – and you keep calling them experts.”

When she had finished, the white-haired chair of the parliament’s economic committee turned to Colau and asked her to withdraw her “very serious offences” in slandering Pellitero. She shook her head and quietly declined.

The “criminal” video became a media sensation, earning Colau condemnation in some quarters and heroine status in others. A poll for the Spanish newspaper El País a few weeks later revealed that 90% of the country’s population approved of the PAH. The group’s work continued. In July 2013, Colau was photographed in Barcelona being dragged away by riot police from a protest against a bank that had refused to negotiate with an evicted family.

Ada Colau calls Javier Rodriguez Pellitero, the deputy general secretary of the Spanish Banking Association, a criminal

Two years later, that image went viral, powered by the extraordinary news that the same T-shirted activist had just been elected the new mayor of Barcelona.

On the day of her inauguration, Colau addressed supporters of all ages gathered on the cobblestones in Plaça Sant Jaume in Barcelona’s old town, thanking them for “making the impossible possible”. Some waved the tricolour of the Second Spanish Republic, which was declared in the very same square in April 1931; its egalitarian ideals buried in the rubble of the civil war five years later.

The date of Colau’s victory – 24 May 2015 – was to be, in the words of one spray-painted graffiti slogan, “a day that will last for years”. Colau had been elected mayor on behalf of Barcelona en Comú, a new “citizens’ movement” backed by several leftwing parties. She became the city’s first ever female mayor, and BComú the first new party to gain power after 35 years dominated by the centre-left PSC and centre-right CiU.

The date was not only significant in Barcelona. BComú was one of several new groups that had defeated the established parties to win power in eight major Spanish cities, including Madrid, Valencia and Zaragoza. These new “mayors of change” became symbols of hope for what progressives in Spain sometimes call la nueva politica.

It has become commonplace across the western world to talk of “new politics” in response to voter apathy, economic crises, corruption and the decline of established political parties. In Spain, however, the phrase has a ring of truth to it. After years of social upheaval following the financial crisis, widespread uprisings against political and business elites have transformed the country’s political landscape. Just as the Indignados, who occupied Spanish squares in their millions in the summer of 2011, inspired the global Occupy movement, it was in Spain, too, that this energy was first channelled into political movements capable of contesting elections, such as the leftwing populist party Podemos.

Colau has been involved every step of the way, and as mayor of the country’s second-biggest city, she now possesses real political power – arguably more so than Podemos, which came third in the Spanish general election last December. The question Colau now faces is a vital one for the left across Europe: can she put her radical agenda into practice?

When I first met Colau last autumn, she was in the middle of an unusual transition, adapting from grassroots activism to life as an elected politician. Having started out at BComú’s spartan office, populated by young people hot-desking on laptops, she was now installed in Barcelona’s 14th-century city hall, with its marble columns, stained glass and Miró statues.

Her calendar had been taken over by a succession of official mayoral duties: gladhanding, exchanging gifts and small talk with dignitaries – death by a thousand micro-ceremonies. The demands on her time are especially intense, since it is central to BComú’s principles and media strategy that the organisation’s figurehead stays on the same level as her supporters, taking public transport and attending neighbourhood BComú meetings where possible.

She cut her own pay from €140,000 to €28,600 and replaced her predecessor’s Audi with a more efficient mayoral minivan

In the weeks following her victory, Colau signalled what might be new about the new politics, with a series of headline-grabbing reforms. “This is the end of a political class removed from the people,” she said, cutting expense accounts and salaries of elected officials. She announced she would reduce her own pay from €140,000 to €28,600, slashed the budget for her own inauguration ceremony, and replaced her predecessor’s Audi with a more efficient mayoral minivan. (She was eventually blocked by political opponents from reducing her salary below €100,000 and has stated that she will donate the remaining sum to local groups.) She suggested withdrawing the annual €4m subsidy to Barcelona’s Grand Prix circuit, restored school meal subsidies to the city’s poorest children, and levied fines worth a total of €60,000 on banks that owned vacant properties. (At the posturing end of the spectrum of political action, she removed a bust of the recent King of Spain, Juan Carlos I, from the city hall’s council chamber.)

She also spent a night out with a homeless charity, helping to count how many people were sleeping rough in Barcelona (almost 900), met mobile phone company workers who were on strike, joined a demonstration against a controversial immigrant detention centre in the city, and returned to speak at the very same local assemblies that had brought BComú to power in the first place.

These initial moves encouraged Colau’s supporters, but the challenge most likely to define her time in office will be taming Barcelona’s tourist industry. In its transformation, since the 1992 Olympics, into the self-styled capital of the Mediterranean, and the fourth-most-visited city in Europe, Barcelona has become a victim of its own success. In the old town, evictions are common – a direct result of rents being driven up by tourist apartments – and residents complain that their neighbourhoods have become unlivable. “You really can’t walk down some streets in the summer,” one local told me, “as in, you physically can’t fit.”

The scale of the problem is made clear by a few simple figures: in 1990, Barcelona had 1.7 million visitors making overnight stays – only a little more than the population of the city; in 2016, the number has risen to more than eight million. In the intervening period, infrastructure and accommodation have been improved and expanded – pavements widened, signage increased, tour buses rerouted – but the problem is a fundamental one. Barcelona is a relatively small city. It is not London, Paris or New York. Major attractions such as the Sagrada Familia and Parc Güell are located in the middle of residential neighbourhoods, not surrounded by the open space they need to accommodate millions of visitors.

Tourism has grown enormously in Barcelona. In 1990 there were 1.7 million overnight visitors; in 2016 there more more than 8 million. Photograph: Dave Stelfox

As tourism has exploded, radically reshaping the city, the question of who Barcelona is ultimately for has become increasingly insistent. “Any city that sacrifices itself on the altar of mass tourism,” Colau has said, “will be abandoned by its people when they can no longer afford the cost of housing, food and basic everyday necessities.” Everyone is proud of Barcelona’s international reputation, Colau told me, but at what cost? “There’s a sense that Barcelona could risk losing its soul. We need to seek a fair balance between the best version of globalisation, and keeping the character, identity and life of the city. This is what makes it attractive – it is not a monumental city, and it is not a world capital like Paris – its main feature is precisely its life, its plurality, its Mediterranean diversity.”

“We want visitors to get to know the real Barcelona,” she said – “not a ‘Barcelona theme park’ full of McDonald’s and souvenirs, without any real identity.” Even in the last few years, the change in Barcelona’s old town is noticeable. The area is no longer dominated by locally owned restaurants, decked with laminated pictures of sangria and tortillas, or little shops selling matador costumes and Gaudí tea towels. Now its narrow cobbled streets are watched over by American Apparel, Starbucks, H&M, Disney and Foot Locker. Every now and then, as you stand in the Barrio Gotic and wonder whether the locals who refer to Barcelona as a “tourist theme park” are being hyperbolic, a bike tour – if you’re particularly unlucky, a Segway tour – will spin around a tight corner and you will have to jump to avoid being body-slammed into an oversized paella dish.

While visitors come for the Gaudí mosaics, al fresco drinking and tapas, there is another side to Barcelona’s culture – a history of barricades, pitched battles with police, and deeply held local neighbourhood identities – that long predates the rise of the tourist industry. In the early 20th century, this rebellious side of the city earned Barcelona the epithet la rosa de foc (the rose of fire). It was there that the radical trade union, the CNT, was founded; by 1919, it had more than 250,000 members in Barcelona alone. That same year, a 44-day-general strike held in the city secured for Spain the world’s first national law on an eight-hour working day.

Colau is not shy about expressing her respect for this heritage. She was born in 1974, in the twilight months of Franco’s dictatorship, only a few hours after the execution of the prominent Catalan anarchist Salvador Puig Antich – an event that Colau has described as formative. Last autumn, she laid a wreath in honour of the anniversary of the execution of Catalan anarchist and educationalist Francesc Ferrer i Guàrdia. It was, she said, thanks to the legacy of figures such as him that she, as an “activist, rebel and Catalan”, could become mayor of the city.

Colau grew up in Barcelona’s Guinardó neighbourhood, playing in the streets with her three sisters and other local children – the idealised Mediterranean upbringing where public space is everyone’s living room. She grew up in a politicised household and participated in her first protests, at the age of 15, against the first Gulf war. She went on to study philosophy at the University of Barcelona and never considered becoming a politician. Later, she studied theatre for a year. When she was 27, she even appeared in a short-lived sitcom about three sisters called Dos + Una – she was the “una”, the eldest of two twins.

It was at the turn of the millennium, as the post-cold-war radical left began to coalesce around a series of anti-globalisation protests in the US and Europe, that Colau became more actively involved in politics. She recalls speaking on the telephone to friends in Genoa during the 2001 anti-G8 protests, after a police raid had left 63 protesters hospitalised. It is this period, she believes, that laid the groundwork for Spain’s new wave of leftwing politics. “I got involved in 2001 with anti-globalisation movements, against the war in Iraq and the World Bank, and global warming,” she told me. “For hundreds of thousands of people, this was the beginning of their involvement with politics, and I still see the influence of this period at work today.”

Colau spent the first years of the new millennium embroiled in activism, protesting and campaigning against wars, poor housing and gentrification. While working for the PAH, she developed her distinctive style of speech, which rests on a sincere, if carefully crafted, populism. She has said that she wants to “feminise” politics and avoids macho or old-left rhetoric. It is hard to imagine her saying, as the Podemos leader Pablo Iglesias once did, that “Heaven is not taken by consensus – it is taken by assault.” Instead, in speeches and interviews Colau returns again and again to a few central themes: human rights and democracy, participation, inclusion, justice. When I used the word “radical” at one point, she challenged it, “But what is radical? We are in a strange situation where defending democracy and human rights becomes radical.”

A key part of Colau’s appeal is that, unlike many politicians, she is not afraid to show emotion. The famous 2013 parliamentary hearing was by no means the only time she has cried, or been close to tears, on camera. At rallies during the mayoral election campaign, she used the whole stage, gesticulating and speaking passionately about the city’s most marginalised residents – women and children and pensioners and migrants and the unemployed – only letting herself uncoil from the performance once it was over and the BComú supporters were on their feet.

In person she is the same, speaking quickly and seriously, not seeming to pause for breath – then, when the message is delivered, she relaxes, often breaking out in laughter. When I met her on BComú’s symbolic 100th day in power, it was the middle of the Merce, Barcelona’s week-long autumn cultural festival. That week it genuinely felt as though the doors of the city hall had been thrown open to the people: normally protected by security guards, the courtyard inside was thronged with festival performers and their families, in traditional Catalan folk costumes of red shirts and white trousers; there were piles of rucksacks on the floor, excited children darting about, and a baby being changed on an ancient oak bench.

Tourists outside the Sagrada Familia in Barcelona. Photograph: Dave Stelfox

From the moment of her election victory, Colau had echoed the Zapatistas by promising to “govern by obeying the people”, and that night she delivered a speech of studied humility. “Never trust in our virtue or our ability to represent you completely,” she told her supporters. “Throw us out if we don’t do what we said we’d do ... but be conscious that we can’t do everything on day one.” It was a response to the paradox at the heart of Spain’s new leftwing politics, which depends upon a small number of charismatic leaders. In Barcelona, for instance, the remarkable collective victory against the establishment by a crowdfunded citizens’ platform, formed only 11 months before the election was built around the appeal of the one woman whose face was on all the posters

In one of her most high-profile speeches of the election campaign, at a rally in September 2014, Colau addressed the grey areas in Spain’s new populism. “They will ask us: ‘Who are you?’ Let’s not be so arrogant as to say we’re ‘everyone’. But we are the people on the street. We’re normal people. We’re simple people, who talk to our neighbours each day, who, unlike professional politicians, use public transport every day, work in precarious jobs every day, and who see how things are every day.” Colau still lives in a modest flat near the Sagrada Familia with her husband Adrià Alemany – with whom she wrote two books about the housing crisis – and her young son Lucas. With Gaudí’s gargantuan basilica at its heart, and three million visitors a year filling the pavements of an otherwise quiet, residential neighbourhood, it is an area that exemplifies Barcelona’s identity crisis.

As Colau has found out, the problem with being the people’s champion, is that not all the people want the same things. In one part of Barcelona’s old town, tensions over tourist excess have spilled over into outright hostility. Tucked away from the sea, Barceloneta’s narrow streets are lined with blocks of flats displaying the barrio’s blue and yellow flag, with a crest featuring a lighthouse and a boat. These days, they are often accompanied by another popular flag, bearing the stencilled Catalan slogan “Cap pis turistic” (No tourist flats).

For centuries, Barceloneta was a traditional working-class fishing district, until the beach on its perimeter underwent extensive regeneration for the 1992 Olympics. The area is now lined with expensive surf shops, rickshaw drivers, sellers of tourist tat and beach volleyballs. Locals complain that the cost of living has shot up and the hordes of tourists often make for bad neighbours.

Tourist misbehaviour peaked in Barceloneta one Friday morning in August 2014, when three exuberant young Italian men spent several hours wandering around the area naked. Photographs of the streaking holidaymakers quickly circulated on social media and a series of anti-tourist protests followed. When I visited last year, the area was plastered with posters put up by the city hall, asking in several languages “Do you know if you’re in an illegal tourist apartment?’ Another in the same series instructed: “Don’t use the street as a toilet.”

As Colau has found out, the problem with being the people’s champion, is that not all the people want the same things

Colau’s stated priority is to move Barcelona away from what she considers “massified tourism”, with no thought for sustainability, strategic planning or input from the public. “Until now, all we have had were private initiatives doing what they wanted,” Colau told me. “This has led to a model that is out of control.” She added: “We suffered the same short-sighted model here with the real estate bubble. We are trying to prevent the same mistakes happening again with tourism.”

Soon after her election, Colau announced a year-long moratorium on new hotels and tourist apartments, disrupting over 30 planned hotel projects. In March 2016, the city hall extended the ban, and is proposing to direct any future expansion to the periphery of the city, away from the over-burdened old town. City hall has also fined Airbnb and its rival Homeaway €60,000 each for advertising illegal tourist apartments – ones that had not been registered and were therefore not necessarily paying taxes or fees. In April, city hall announced it was looking into a specific tourist tax levied on those not making overnight stays: cruise ship passengers and day-trippers. Many of these initiatives have come from Ada Colau’s new tourism council, which features input from ordinary Barcelonans, as well as the industry.

Even so, many locals are still unhappy. On the first day of the Merce, as the crowds gathered in Plaça Saint Jaume for Colau’s ceremonial opening of the festival, the Barceloneta neighbourhood association staged a protest. The locals, many of them accompanied by young children, faced the city hall waving blue and yellow flags, banging drums and blowing whistles. “Life in Barceloneta has become unbearable,” Kico Casas bellowed to me above the din. He and his fellow activists are campaigning for a total abolition of tourist flats in Barcelona. “Speculation has led to so many rent rises,” said Casas, “and now we can’t afford to live in the neighbourhood our grandparents lived in. Meanwhile, the drunken tourists and their parties make ordinary life unbearable.”

On a demonstration the previous week, the Barceloneta neighbourhood association had marched to the city’s Airbnb offices, wheeling a cannon alongside them – a theatrical homage to the area’s marine heritage – and fired a fake shot at the apartment rental company. On that occasion they had singled out Colau, too, with a homemade banner reading: “Mayor: three months without solutions. Well?” It was the first time Colau’s core supporters, or at least one strand of them, had faced up to their champion.

On the other side of the old town from Barceloneta lies the Raval, another area with a long history of poverty and strident working-class solidarity. One afternoon, I attended a community discussion event here, which took place on ground where a factory once stood. The empty plot was due to have a luxury hotel built on it – instead, the site was occupied by local activist groups who had turned it into a “social space”, covered in graffiti art decrying police brutality and city branding of the “I <3 Barcelona” variety. A man named Manel Aisa took the mic to explain that he grew up on this very street in the 1950s, where his dad ran a bar populated by duckers and divers, radicals and sex workers. He explained that the week before, he had been walking through the Raval, when a group of young German tourists approached him and asked in faltering Spanish, “Is this a good area to invest in property?” He managed a laugh, recalling the cheek of the question. “I told them where to go – away.”

Apartments in Barceloneta display banners protesting against tourist flats. Photograph: Dave Stelfox

But the difficult truth is that for many Barcelonans – not just a wealthy elite of cruise ship owners, hoteliers and landlords – the tourist economy has been a source of salvation. “For the majority of people sharing their home, it’s about making ends meet,” Ricardo Ramos, spokesman for the Barcelona Association of Neighbours and Hosts, explained over lunch near Sagrada Familia. “We have pensioners who are trying to pay the mortgage, or the rent, and live on €400 a month – and that’s impossible in Barcelona. Some of these people would be on the streets within two months, without that extra income.”

Ramos’s organisation, which was founded in April last year, is supported by Airbnb. Its members have organised their own protests – with slogans, written in English, such as “Tourists come home!” , instead of “Tourists go home”. Ramos explained that, as well as helping home sharers, the type of tourism encouraged by companies such as Airbnb generates income for small shops located outside the obvious tourist centres, and provides a more local and authentic experience than a fleeting walk around La Rambla and a night’s sleep in an international hotel chain. Airbnb points to a 2014 study that found that more than half of the company’s Barcelona hosts had used the platform to help pay their mortgage, rent or bills – in the process, generating €128m and creating more than 4,000 jobs in the previous year.

Like his opponents in Barceloneta and the Raval, Ramos argued that if Colau were really of the people, she would be supporting them: “Given that Mayor Colau comes from a socialist background, I don’t understand why empowering citizens to take action to avoid being evicted from their homes is so difficult to understand. We should be on the same side. Home sharing and tourism has been stigmatised in Barcelona – some groups of neighbours have been out on patrols, at night, to see where the tourist flats are. And Mayor Colau doesn’t stop it.”

“I think Mayor Colau doesn’t understand the difference between being in an election campaign, and being in power,” Ramos continued. “When you are in the campaign, you talk to your audience, that’s fine – but once you are in power, you rule for all citizens, regardless of whether they voted for you or not.”

With a minority government of only 11 of 41 councillors, Colau and BComú have required support from other parties to get new legislation passed. They have also faced hostility from the business community and media – not to mention an intransigent local bureaucracy. The threat that BComú’s enemies posed to stable governance was clear from the outset – even before the mayoral inauguration, Jean Delort, the political representative for the Barcelona police, resigned in protest at the election of Colau. “For them, there are no decent police,” said one police spokesman. “We’re all torturers.”

BComú has encountered substantial opposition in the council chamber from established parties keen to block its more radical reforms and expose its inexperience. In October, two parties which were nominally allied with BComú – the centre-left PSC and the leftwing Catalan nationalists ERC – voted to reverse Colau’s moratorium on new hotel building (it was renewed in March nonetheless). The following month, the PSC leader Jaume Collboni described the measures as “indiscriminate”, accusing Colau of ideological purism and “profound ignorance of the terrain” in “a complex city like Barcelona”. He proposed that the novices in BComú would benefit from his party’s governing experience, and that only a co-governing pact with the PSC would stabilise the “extreme weakness” of Colau’s administration.

On 10 May, Colau finally relented, and announced just such a pact. BComú will be bolstered by the PSC’s four councillors, and Collboni will become deputy mayor. Perhaps more importantly than votes in the city council chamber, the PSC will give Colau’s administration access to a network of contacts, which includes influential bureaucrats, union officials, commercial and civil society associations.

As with any governing coalition, behind-the-scenes politicking and media spin will be vital in determining which party is judged to have been the “winner” from the deal; in the short term, it is hard not to see it as a defeat for Colau. In January 2015, four months before the election, she had ruled out just such a pact with the PSC, whom she called one of the “parties of the regime”, and as such, “part of the problem, not the solution”.

A stencil of Mayor Ada Colau in the Gracia neighbourhood of Barcelona. Photograph: Dave Stelfox

Some activists are sceptical about what this compromise will do to BComú; according to an article published last week in the leftwing newspaper Diagonal, the PSC pact is “like getting a dominatrix into bed, with the hope they will assume a submissive role”. Events in Barcelona in the last few days risk further alienating some of Colau’s core supporters. The eviction of squatters from a former bank that they had turned into a social centre led to violent clashes with riot police. She angered some people by refusing to get involved in what she said was a private dispute (although she has also offered to find the squatters an alternative site).

These kinds of setbacks raise a bigger question for BComú supporters and those of other new parties such as Podemos: was it all worth the effort? Might they just have been better off lobbying for change from outside their various parliaments?

For some experienced observers, taking activist politics into the institutions of power was always going to be a challenge. Oriol Nel·lo is professor of urban geography at the Autonomous University of Barcelona, and a former PSC representative in the Catalan regional parliament. In last year’s municipal election, he backed Barcelona en Comú. Grassroots activists should not think of city hall “as a fortress”, he told me over coffee in a cloistered square in the Raval. “It’s better to think of it as a very complex arena, in which you can manage to conquer certain positions – knowing that these institutions are more likely, a lot of the time, to give way to other pressures, coming from the economic sector or from business. But that doesn’t mean you can’t do anything within the institutions,” he smiled. “You can change plenty of things.” For Nel·lo, Colau’s determination to rebalance the effects of tourism in favour of Barcelona’s citizens is one example of a reform that is both essential and achievable.

In the summer of 2016, Spain’s political scene is in a strange purgatory: the old is dying and the new cannot be born. The rise of new parties on the left and right culminated in an inconclusive general election in 2015, without a decisive victor. Six months of coalition talks resulted in stalemate, and so Spain will go to the polls again at the end of June; the results are likely to be equally unclear. In the meantime it is Ada Colau, and her fellow mayor in Madrid, Manuela Carmena, who remain the most powerful proponents of ‘the new politics’ in Spain – a country where, despite a short period of economic recovery, unemployment remains above 22%.

Even after the compromise with the PSC, there is a sense among her supporters that Colau’s experience fighting for housing reform, occupying banks and blocking evictions with the PAH has given her the confidence and perseverance to see the project through. It is, she told me, “a collective made up of the poorest people in Spain, people who have lost everything – not just their homes, or their money, but their hopes for the future.” With nothing left to lose, they got organised, formed close bonds, supported new friends, joined in civil disobedience together, fought and kept fighting – and they won. “It’s an experience I will never forget in my entire life,” Colau said, “because it taught me the most valuable lesson I have ever learned, which is that we will be whatever we want to be. To have a society that is more just truly depends on us, and on whether we get involved or not.”

Main photograph by Dave Stelfox

• Follow the Long Read on Twitter at @gdnlongread, or sign up to the long read weekly email here.