Anti-austerity protesters clashed with riot police in Madrid late Saturday and early Sunday, resulting in several injuries and arrests.
Activists threw rocks and bottles at police, who responded with rubber bullets.
A month of anti-austerity protests culminated in a massive march in the Spanish capital.
Tens of thousands of people came from across the country for a largely peaceful demonstration against deep austerity measures.
Violence erupted toward the end of the rally, when some protesters attempted to break through a police barrier and make their way to the nearby headquarters of the governing conservative Popular Party.
At least six officers were injured and that 12 people were arrested, according to police.
The rally was staged to denounce the government, which has been accused of injecting millions of euros to help bail out struggling banks rather than helping the Spanish population.
Labour reforms are said to be partly to blame for an unemployment rate of over 25 per cent.