Until this week, the images habitually projected by the Catalan independence movement were of its red, yellow and blue estelada flags and of the huge crowds that have gathered on the region’s national day over the past eight years to make earnest, enthusiastic – and fruitless – calls for a separate republic.

By Monday night, however, the pictures coming out of Barcelona and elsewhere had begun to tell a different story. Infuriated by the Spanish supreme court’s decision to jail nine pro-independence leaders over their roles in the failed push for secession two years ago, thousands of young Catalans marched on Barcelona-El Prat airport in an attempt to occupy it.

Police responded with baton charges and fired rubber and foam bullets, and the clashes have been followed by three further nights of unrest in the region. Cars have been torched in central Barcelona, barricades set ablaze, buildings vandalised and police officers attacked with petrol bombs, fireworks and stones.

For some pro-independence Catalans, patience with traditional forms of protest appears to be running out. The court verdict compounded resentment still felt over the Spanish police’s brutal attempts to halt the unilateral independence referendum in October 2017.

Tellingly, the airport occupation – with its echoes of the enduring protests in Hong Kong – was not called by the two biggest traditional pro-independence civil society groups, the Catalan National Assembly and Òmnium Cultural. It was the brainchild of a secretive new group called Tsunami Democràtic that is using apps and social media to control and co-ordinate the protests.

Tsunami Democràtic announced itself across Twitter, Instagram and Telegram on 2 September this year. “We are not a new organisation, we are a continuing and inexhaustible campaign,” it said at the time. “We can fight back. There’s a strategy. A wave is beginning and you are the protagonist. You are the Tsunami.”

The group, which was swiftly endorsed by the former Catalan president Carles Puigdemont, has been among those opposed to the violence of recent days. “We never see violence as an option, primarily because it leads us away from our objectives,” the group said in an emailed interview with the Guardian. “Non-violence is the winning strategy and that’s why we follow it.”

Proof of the group’s reach came on Friday afternoon when a judge at Spain’s highest criminal court, the Audiencia Nacional, ordered police to take down Tsunami’s website and social media accounts as Barcelona braced for the biggest demonstrations of the week.

Tsunami said it had three basic aims: the exercise of the right to Catalan self-determination, freedom for the jailed and exiled prisoners, and the full exercise of fundamental rights. “On the matter of achieving those aims, our demand is simple and direct: Spain, sit and talk.”

Protesters clashed with police outside El Prat airport in Barcelona on Monday. Photograph: Pau Barrena/AFP via Getty Images

When news came through of the sentencing of the Catalan prisoners, Tsunami used Twitter, Telegram and word of mouth to get people to the airport.

It has more than 320,000 subscribers on Telegram and rolled out a new app of its own after the airport occupation. To access it, the user needs a QR code that it scans from another, trusted user. So far, more than 15,000 people have downloaded the new app, which has yet to be used in any action.

Mondays calls were swiftly answered by many young Catalans. Irma, 19, was in class when she got the message. Marc and Anna, both 21, were at a demonstration in Plaça de Catalunya in central Barcelona.

“We were hanging around in Plaça Catalunya and we didn’t have any idea what to do when we got the message to occupy the airport,” Anna told the Guardian.

“The plan was to paralyse the airport to get across our message that we’re not happy with what’s happening,” said Marc, who, similarly to many others, reached the airport after a five-hour journey by public transport and on foot.

Many had been able to download fake boarding passes for existing flights via Tsunami to enable them to cross police lines. “At first there was no violence where I was, just some people sitting on the ground,” said Irma. “Then suddenly the police starting beating people with their truncheons.”

Anna managed to get inside the airport via the roof and made for the parking area before the police began charging. “We hid behind the buses,” she said. “The police were firing down on the crowd from above.”

Tsunami Democratic said that like the Hong Kong protesters, it believed in Bruce Lee’s famous mantra “Be water, my friend”. It added: “It’s about knowing how to adapt, creatively and generously, to each new circumstance.”

Wednesday night’s march was called by the direct action group Committees for the Defence of the Republic (CDR). It is not clear how much this group overlaps with Tsunami.

Tsunami said it had nothing to do with the CDR and said some people found it difficult to understand the “enormous degree of collective will” within the Catalan population. The group confirmed it had had one-off contacts with pro-independence parties and groups, although “never with the government of [the Catalan regional president] Quim Torra”.

According to Anna, Tsunami is not hierarchical. “It’s more about people who trust each other taking action,” she said. “I don’t know if it’s linked to the CDR. Probably, because it’s clearly people with experience in organising things. No one really knows. It’s more that people like me and my friends share information.”

Protesters hold up their mobile phones during a demonstration in Barcelona. Photograph: Lluís Gené/AFP via Getty Images

Some think Tsunami may have been formed in an attempt to detoxify the CDR brand after seven people associated with the committees were remanded in custody after being accused of belonging to a terrorist organisation and possessing explosives.

“The CDR acquired a negative image – especially when the police investigated and found some of its members with explosives,” said Verónica Fumanal, a political scientist and political communications specialist who has worked for Spain’s socialist prime minister, Pedro Sánchez, and for Albert Rivera, the leader of the centre-right Citizens party.

“From that moment, the independence movement couldn’t allow itself to be seen as a movement associated with any kind of violence. Something new was needed, a new actor that could mobilise people.”

Fumanal pointed out that the independence movement had always been extremely good at getting its message across, but argued that Tsunami Democràtic was a misnomer. “They talk about civil disobedience, but that’s like calling an economic crisis a recession,” she said. “You skirt the negative fact that this is neither a tsunami nor democratic. A tsunami sweeps away everything in its path. But what we’re seeing here are 20,000 very well-organised people in Catalonia.”

Fumanal also wondered whether the violence of recent days was proof that Tsunami was losing its ability to keep some people on the path of non-violence. During Wednesday’s rioting, groups of young people threw bottles and stones at the police while chanting “som gent de pau” (we’re peaceful people).

“Obviously you can’t avoid the kind of violent scenes we’ve seen because that’s what happens when you’re dealing with a crowd,” said Fumanal. “But there’s a difference between a spontaneous act of violence and generating the kind of massive chaos we saw in Barcelona on Wednesday or at the airport on Monday.”

Spain’s acting interior minister, Fernando Grande-Marlaska, said intelligence services were looking into Tsunami and who was behind it.

Pacifism has long been the independence movement’s trump card but after seven years of peaceful protest with little or nothing to show for it, frustration is giving way to acts of violence.

Many thousands of Catalans are marching peacefully along the region’s roads and motorways to congregate in Barcelona for a huge protest on Friday. However, others feel the road to independence has already proved too long and tortuous.

“It would have been really lovely if the peaceful approach had worked but revolution – if that’s the right word – has never achieved its goals without violence,” said Anna. “It’s sad that our only reward for years of peaceful protest was to get beaten up.”

• This article was amended on 19 October 2019 to remove a paragraph about an action near Barcelona’s former bullring. Tsunami says the only action it co-ordinated was the occupation of the airport on Monday. It was also amended to clarify how people were mobilised.