People were wondering what had happened to the "Indignants", the protesters who swamped Spanish squares in 2011 to demand political change.

Podemos, a left-wing party that emerged out of the Indignants, won eight percent of the vote in last weekend's European elections, giving them five seats in the European Parliament.

Although they still have a long way to go to really trouble Spain's establishment, the result took many observers by surprise since opinion polls had forecast only a two or three percent vote share for the party.

It was particularly impressive since the party was only officially formed four months ago, and contributed to the decline of the mainstream Socialist party, whose leader, Alfredo Perez Rubalcaba, resigned in the aftermath.

"It is a party with little structure, largely based on television appearances," said Ignacio Molina, a specialist in European politics at the Real Instituto Elcano research group.

"It is very active on the social networks online. It is a party of new technology and young people, but a very important one," he added.

"It has managed to channel the protest movement and the hopes of the Indignados."

At the helm of Podemos, whose name means "We Can", is its earringed, ponytailed lead candidate Pablo Iglesias.

A regular face on political talk shows, the 35-year-old university politics lecturer won over more than a million voters in Spain with his campaign against what he calls the established political "caste".

He echoes the cries of the mass street protests that rocked Spain in 2011 and 2012, condemning corruption and the financial powers that be.

"We are being governed by the menservants of the rich," he was quoted as saying by El Mundo newspaper this week, reiterating a regular line.

"What we need is a government of postmen of the people. Our party's aim is to become an alternative" to the political class.

"He is an outstanding person. He has a populist way of speaking, simple and effective," Molina said of Iglesias.

Podemos's unexpected success raised debate about whether Spain's old two-party system was under threat.

The country has been governed alternately by conservative and centre-left parties since democracy was restored in the late 1970s.

The governing conservative Popular Party and opposition Socialists each saw their share of the vote and seats plunge in the EU elections.