Five years ago, a small group of angry activists launched Spain's anti-austerity Indignados movement, likely never imagining it would grow so big, spreading beyond the country and shaking the very core of politics.

In the spring of 2011, as an economic crisis tore through Spain leaving countless jobless and homeless in its wake, a dozen bloggers and online activists decided to write a manifesto titled "real democracy now" and called for a demonstration.

Under a clear blue sky on May 15, thousands responded to the call, gathering on the Puerta del Sol square in Madrid where they set up camp, refusing to budge for weeks just like Egyptian youth had done on Cairo's Tahrir square.

With more than 20 percent unemployment, floods of exasperated people around the country followed suit, taking to the streets and organising meetings in what became known as the Indignados -- or 15-M, short for May 15 -- movement.

"It was a major event, which resulted in a new political and historical era in Spain," says sociologist Jaime Pastor.

- 'Instrument for change' -

Initiated when the Socialists were still in power, the Indignados claimed to transcend political cleavages.

"Some of us are more progressive, others more conservative... But we are all worried and 'indignant' about the political, economic and social situation," read their manifesto.

That year, 21,000 protests took place throughout Spain.

And the movement quickly spread beyond Spain, inspiring Occupy Wall Street protests in New York as well as gatherings in other European cities.

As the crisis deepened, Spaniards chose to vote in the conservative Popular Party (PP) in November 2011, chasing away the Socialists.

The PP continued and deepened a policy of spending cuts and tax rises put in place by the previous government in a bid to steer the country away from economic collapse.

They achieved that, bringing Spain back to growth, but at a cost.

Austerity measures increased inequalities and a string of corruption scandals that hit the PP only deepened resentment of the elite.

And so it was that Podemos, a far-left, anti-austerity party inspired by the Indignados movement, was born in January 2014.

Criticised as a populist, anti-establishment grouping by its detractors, the ally of Greece's Syriza leftists nevertheless swept into third place in general elections less than two years later.

In a historic move, the December 2015 polls put an end to Spain's long-standing two-party system thanks to the rise of Podemos as well as that of Ciudadanos, another new grouping considered more to the right, leading to a hung parliament.

A new political generation was born: out of 350 lawmakers, 211 had never held a parliamentary seat -- suited men and women contrasting with those sporting jeans and dreadlocks.

The Indignados movement also filtered into city halls in major cities like Madrid and Barcelona, with activists now holding prominent positions after municipal elections in May last year.

Ada Colau, the current mayor of Barcelona, rose to prominence as an activist who protested against forced evictions.

"Politics has once again become an instrument for change," says Podemos lawmaker Juantxo Lopez de Uralde, the former head of Greenpeace in Spain.

- 'Still a lot to do' -

But this sweeping change has yet to make an impact.

Coalition talks to form a government collapsed after bickering parties failed to come to any form of agreement, leaving the country in political limbo.

As a result, Spaniards are having to go back to the polls in June in the first repeat elections since the country returned to democracy following the death of dictator Francisco Franco in 1975.

In the meantime, Spain is still being ruled by a caretaker conservative government blamed for the very austerity that the Indignados movement wants to quash.

"There is still a lot to do," acknowledges Fabio Gandara, one of the founders of the movement.

"The majority of our demands have yet to be fulfilled."

Still, according to Podemos activist Tristan Duanel, there have been some achievements.

"Young people have joined parties," he says, adding that some themes advocated by the Indignados -- such as transparency -- have now become mainstream.

"But change is slow," he adds. "We have to do something at a European level."

Pastor believes that is beginning to happen, with initiatives such as the Plan B Europe-wide anti-austerity movement created by former Greek finance minister Yanis Varoufakis, or the youth-led "Nuit debout" protests in France.

"We are going back to transnationalism," he says.