Allegations of illegal cash payments to members of Prime Minister Mariano Rajoy's ruling Popular Party have outraged Spain amid rising unemployment, emigration to Latin America in search of jobs and continuing cutbacks in public and private sectors.
Spain's property bubble triggered the initial financial crisis but drastic spending cuts ordered by the eurozone regulators in Brussels have exacerbated public hardship. Reports of corruption in high places, some implicating Rajoy, were seen by protesters as the last straw.
Rajoy has denied receiving any illegal payments but persistent accusations continue, some fanned by the opposition.
Protesters marched on Popular Party headquarters in central Madrid and many clashed with police who stood guard over the building. Organizers have promised more protests for the weekend.
Marchers swelled in number and Spanish media said some protests drew more than 1,000 participants to rallies in central Madrid, causing traffic chaos.
Protests also took place in some of the regional centers.
Rajoy has seen his approval ratings plummet as public anger, previously focused on Germany and the eurozone for demanding harsh austerity cuts, has homed in on his beleaguered government. The corruption scandal has damaged public confidence in the government.
Despite the mounting criticism of alleged government involvement in illegal payments, progress on securing prosecutions has been slow.
Protesters want more ruling party senior officials to answer charges and are not content with an ongoing case brought against former Popular Party treasurer Luis Barcenas. Rajoy and his senior aides were named by Barcenas in connection with alleged cash payments. The payments were funded from slush funds managed by the party after numerous illegal "donations" by businesses.
Protesters are outraged the businesses paid out vast sums for the politicians while getting rid of employees. Rajoy and aides deny receiving any cash payments and accuse Barcenas of attempting blackmail.
Analysts said the allegations against Rajoy put government austerity reforms on the back burner.
The opposition has orchestrated a vigorous campaign for Rajoy's resignation.
El Mundo newspaper said the documents it had seen purport to show Rajoy received payments in 1997, 1998 and 1999 when he was a minister in the government of Jose Maria Aznar.
The payments included two to Rajoy for a total of $16,800 in 1998.
The newspaper also published text messages indicating alleged links between Rajoy and Barcenas.
Barcenas said the photocopies originally published by El Pais newspaper were a fraction of the documents he had in his possession.