Thousands rallied in Madrid on Saturday to support a no-confidence vote against conservative prime minister Mariano Rajoy brought by the far-left Podemos party.
Podemos organised the gathering to bolster its no-confidence vote against Mr Rajoy's ruling Popular Party, which has been hit by a series of corruption scandals.
The rally under the slogan "We have to throw them out" was held in the Puerta del Sol, a large square in the heart of Spain's capital.
Many protesters held signs that read "Enough!" or "Corruption!"
Podemos registered its intention on Friday to bring the no-confidence vote to Parliament.
The move includes presenting the party's pony-tailed leader, Pablo Iglesias, as an alternative candidate to replace Mr Rajoy.
No date has been set for the no-confidence vote but the move appears designed to fail.
With only 71 members in parliament, Podemos would need help from other parties to reach the majority needed of 176.
No other major party says it will back the move to topple Mr Rajoy.
Speaking at a Podemos party congress before the rally, Mr Iglesias admitted that "the no-confidence vote won't prosper".
But hours later, Mr Iglesias struck a defiant tone at the rally, calling the Popular Party "a mafia-like party".
"The people are not afraid. They are telling the corrupted to 'get lost, we want a Spain of the 21st century," Mr Iglesias said.
"This country is better than its parliament and we are showing the way to the future."
Mr Rajoy has been dragged into the most damaging of corruption cases involving the Popular Party, an alleged kickbacks-for-contracts scheme to finance party activities.
Spain's National Court has called Mr Rajoy as a witness in the case.
Like his party, Mr Rajoy has denied any wrongdoing.
He has asked to appear for the court hearing via video conference in July.
On Monday, Podemos will present a motion to hold a separate no-confidence vote against Madrid's regional leader, Cristina Cifuentes, for another corruption investigation involving the Popular Party.
Podemos was founded in 2014, partly channeling the "Indignados" protest movement of 2011 that protested the impact of Spain's financial woes during the European financial crisis.