A few days ago, two words in a splash of red appeared beneath the first-floor window of the art deco building at 18 Patriarchou Ioakim Street. Their message was unambiguous. “Everybody Syriza”, read the scrawl.

Anywhere else the graffiti might have gone unnoticed. But Patriarchou Ioakim is the central artery that bisects Kolonaki, perhaps the most gentrified and upscale district in central Athens.

Elina Papadatou, who works at Dolce Vita, the high-end confectioner’s opposite, confesses to being quite shocked when it suddenly appeared, as do the young women selling made-to-order evening dresses and wedding gowns – more than €3,000 apiece – in the showroom of the atelier behind the first-floor window. “After all, you couldn’t call Kolonaki a bastion of the left,” says Papadatou. “If anything, I’d say exactly the opposite.”

But as Greeks vote on Sundayin an election whose ramifications will be felt far beyond the borders of their country, quite a few in Kolonaki will not be supporting the conservative New Democracy party. Throwing traditional preferences to the winds, they will be voting for Syriza, the radical left anti-austerity party on course for victory and taking Europe by storm.

All over Greece, in middle-class enclaves eviscerated by the effects of austerity, others will be doing the same. “The bourgeoisie is going to play a major role in securing Syriza’s forthcoming victory,” says political scientist Dimitris Keridis, adding that wealthy friends, frustrated by exorbitant property taxes, have joined the band of turncoats. The exodus will be a repeat of the desertion the conservatives suffered when one in 10 voters migrated to the left in last May’s Euro-elections.

After five years of being subjected to the economic brutalities of neoliberal orthodoxy – the price of the greatest bailout in global financial history – Greeks are not the people they once were. Change is everywhere: in politics, financial affairs, the social fabric and states of mind. This election will be the biggest shift of all: a historic turning point in a country that, on the frontline of the euro crisis, has defiantly challenged the prevailing narrative from Brussels and Berlin.

The leader of Greece’s leftwing Syriza party, Alexis Tsipras. Photograph: Louisa Gouliamaki/AFP/Getty Images

Under the charismatic Alexis Tsipras, the radical leftists, who have been leading polls for the best part of a year, are on a mission. A once-derided political force is now determined to rock the foundation of Greek politics and, with it, Europe too. As the chief architect of austerity, Angela Merkel, the German chancellor, should be watching with justifiable nervousness.

For Professor Constantine Tsoukalas, Greece’s pre-eminent sociologist, there is no question that, come Monday, Europe will have reached a watershed.

I first met Tsoukalas in January 2009, in his lofty, book-lined apartment in Kolonaki. For several weeks Athens had been shaken by riots triggered by the police shooting of a teenage boy. The violence was tumultuous and prolonged. Looking back, it is clear that this was the start of the crisis – a cry for help by a dislocated youth robbed of hope as a result of surging unemployment and enraged by a system that, corrupt and inefficient, favoured the few.

Tsoukalas knew that this was “the beginning of something” although he could not tell what. But with great prescience he spoke of the degeneration of politics – both inside and outside Greece – the rise of moral indignation, and the emptiness of a globalised market “that was supposed to put an end to ideology but, in crisis, has instead created this moment of great ideological tension”.

Six years later, following the longest recession on record, he is in little doubt that anger has fuelled the rise of Syriza. On the back of rage over austerity, the leftists have seen their popularity soar from 5% before the crisis to as high as 35% – more than the combined total of New Democracy and left-leaning Pasok, the two parties that have alternated in power since the restoration of democracy in 1974.

Neo-Nazi Golden Dawn, the group that has been the other beneficiary of despair – but whose support has dropped amid revelations of criminal activity – may yet surprise if it succeeds in coming in third.

“The European policy towards Greece, to a large extent, has been determined by the will to experiment with the feasibility of shock therapies,” says Tsoukalas. “It worked, but the reaction is going to be a leftwing government. Europe cannot survive as it is. The rise of fascism … should be sufficient [evidence] to everyone that it has to change.”

If Greece’s rebellion was to occur in a coherent way, Tsoukalas, who is being fielded by Syriza as an honorary candidate, believes it would be only a matter of time before it was replicated in other parts of the continent. “These elections are important because they are a reminder to the people of Europe that there is another way out,” he insists. “That neoliberal orthodoxy is not an immovable problem.”

The business community, no ally of the left in a nation where communists were hounded for much of the 20th century, is bracing itself for the inevitable. But it has found it hard to conceal its anxiety. With bailout funds guaranteed only until the end of February – and negotiations with creditors at the EU, ECB and IMF still stalled over the need for further austerity – there are fears of a bank run, or worse.

Under the chandeliers in the hotel where the American-Hellenic Chamber of Commerce held its new year party last week, the financial elite was in an anything but festive mood. Simos Anastosopoulos, the chamber’s president, expressed fears over the country’s liquidity: bank depositors had been withdrawing a rumoured €1bn a day last week.

There was mounting concern that the economy would simply collapse if an agreement was not reached with lenders soon. “The government has liquidity of about €2bn but has obligations [in maturing debt] in the next few months that far exceed that,” he told me. “Syriza says we have enough money until June, but that is not the case. If negotiations are dragged out because of arguments over reforms, the damage to the banking system might have been done.”

But at 28 Veikou Street, not far from the Acropolis, such abstractions are of little concern. Here Syriza runs the Solidarity Club – initially set up as a food bank in March 2013 when stories began to surface of malnourished children fainting in schools. In recent months, its staff have focused on providing medicines.

“That’s the big problem now because so many are uninsured, without any access to the health system,” says volunteer Panaghiota Mourtidou. “People don’t have the money to go to doctors. If they have a toothache, they get terrified, because how the hell are they going to pay for a visit to the dentist?”

With its Che Guevara posters, Italian Euro-communist flags, chaos of boxes and tins, and makeshift furniture, there is something of a field-camp feeling about Veikou Street. But its army of volunteers are tireless. This, they say, is a battle to be won, a huge victory for the left that Greece will set in motion.

“We are conscious that we have managed to unite in a way that the left elsewhere has failed to do,” says Angeliki Kassola, a theatre director, drawing on her freshly rolled cigarette. “I’ve met lots of once-strident New Democracy supporters who say they will be voting for us because they are attracted to Syriza’s vision of democracy, justice, dignity – all the things that have been taken from us in the crisis.”

A steady stream of foreign sympathisers – from Spain, Italy and France – visited last week. Vincent Vetori and his wife Anne, who flew in from Paris to be in Athens for the vote, walk in, leaflets in hand. They have spent their Friday evening plastering the neighbourhood with posters bearing Syriza’s motto: “Hope is Coming.”

“Syriza is our hope in Greece,” says Vincent, 53, an entrepreneur. “In France the word ‘leftist’ has been stolen by a government that calls itself socialist but enacts rightwing policies. We want Syriza to win to help change the dynamic and create hope in Europe.”