Tens of thousands of people have massed in central Madrid for a rally organised by radical Spanish leftists Podemos.
The "March for Change" is one of the party's first outdoor mass rallies, as it looks to build on the recent victory of its close allies Syriza in Greece.
Podemos leader Pablo Iglesias told the crowd a "wind of change" was starting to blow through Europe.
Podemos has surged ahead in opinion polls, and has vowed to write off part of Spain's debt if it comes to power.
The BBC's Tom Burridge in Madrid says that there has been an impressive turnout and a carnival atmosphere at the rally.
Several of Madrid's main avenues became a sea of people and purple, the party's colour, he says, after its supporters travelled from all over Spain.
Marching from Madrid city hall to the central Puerta del Sol square, protesters shouted "Si, Podemos!", meaning "Yes, we can".
Broadcaster TVE reported that hundreds of thousands were at the demonstration, but there was no official tally.
"The wind of change is starting to blow in Europe," Mr Iglesias said, addressing supporters in Greek and Spanish at the start of the rally.
"We dream but we take our dream seriously. More has been done in Greece in six days than many governments did in years."
Protesters are parading in the same streets that over the past six years have seen many other gatherings against financial crisis cutbacks imposed by successive governments.
One marcher, Jose Maria Jacobo, told Reuters news agency that people had to fight back against the political class.
"It is the only way..., to kick out all of those politicians who are taking everything from us. They even try to take our dignity away from us. But that they won't take that from us," he said.
But speaking in Barcelona, Spanish Prime Minister Mariano Rajoy said Podemos had no chance of winning elections.
"I don't accept the gloomy Spain which some want to portray because they think that by doing so they will replace those who are governing and have had to face the most difficult crisis in decades. They will not succeed," he said.
Many Spaniards are enraged over reports of political corruption and public spending cuts implemented by Mr Rajoy's People's Party and before that by the Socialists.
The two big traditional parties have described the party - less than a year old and whose names translates as "we can" - as populist.
Our correspondent says that since Podemos stormed onto the political scene in last May's European elections, it has moved from strength to strength with its uncompromising message against austerity and corruption.
But both left-wing and right-wing media have criticised Podemos, accusing it of having ties with Venezuela's left-wing leaders and alleging financial misconduct by some of its senior members.
The party's leaders have in response promised to publish their tax returns, with Mr Iglesias remaining defiant.
"In the face of their hatred, we smile," is one of his regular pronouncements, according to the AFP news agency. After the Syriza triumph in the Greek elections he said that "hope had been born".
Spain has now officially come out of recession but nearly one in four workers remains unemployed.
Last year was the first time there has been full-year economic growth in the country since 2008, when a property bubble burst, putting millions of people out of work and pushing the country to the brink of a bail-out.