Thousands of protesters clashed with riot police in Madrid as they called for the resignation of Prime Minister Mariano Rajoy.
The protesters packed streets outside the headquarters of the ruling Popular Party, insisting that Rajoy should leave office because of allegations he received payoffs from a slush fund before his party won elections in 2011.
What started as a peaceful protest turned violent just before midnight, leaving several people injured and resulting in a number of arrests. Riot police used batons and there were reports of at least one officer being injured by missiles thrown from the crowd.
The demonstrations came after opposition leaders this week called for Rajoy to explain himself before parliament or face a censure vote.
Thousands more people also demonstrated in Barcelona.
On Monday Mr Rajoy tried to brush off demands that he resign after text messages were leaked which were claimed to show his support for the under-fire former political party treasurer Luis Barcenas.
Mr Barcenas is currently under investigation over separate charges relating to corruption and secret Swiss bank accounts and has made allegations that Mr Rajoy also took illegal payments – which the Prime Minister denies.