Spain's opposition leader called on Prime Minister Mariano Rajoy to resign today after he was embroiled in a corruption scandal.
Mr Rajoy publicly denied taking backhanders on Saturday, prompting violent clashes between police and impromptu protesters.
He also promised to publish details of his financial dealings in a bid to stop the disquiet.
Newspaper El Pais published on Thursday details of secret papers belonging to Mr Rajoy's former Popular Party treasurer Luis Barcenas.
They allegedly show that businesses were channelling payments to Mr Rajoy and other party leaders.
El Pais said the then junior minister received around €25,000 (£22,000) a year between 1997 and 2008.
He denied the allegations: "I have never received or handed out 'black money' in this party or anywhere else."
But the public were unconvinced.
Police cordoned off several of Madrid's main roads on Saturday night to stop thousands of spontaneous protesters gathering in large groups.
Scuffles broke out in Madrid, Barcelona, Alicante, Valladolid and Seville as police kept protesters away from Popular Party offices.
Protester Miguel Gomez said: "It's absurd. They're telling us these payments don't appear in the books, but of course they don't.
"They're having a laugh at our expense."
The allegations come as Mr Rajoy faces growing unrest over unpopular austerity measures intended to rescue the country's ailing economy.
Data released last week showed that fourth-quarter output for 2012 was down 1.8 per cent on the year before and the worst since the 2009 crisis hit.
And unemployment has hit a post-dictatorship record of 26 per cent.
Socialist Party leader Alfredo Perez Rubalcaba said: "Rajoy should give up his role as the head of government (because) he cannot tackle the very difficult situation confronting Spain."
A weekend poll showed that support for the People's Party had plummeted to 23.9 per cent, having won 2011 elections with 39.9 per cent of the vote.
Around 77 per cent of respondents said they didn't want Mr Rajoy as head of government, with 85 per cent saying they had little or no faith in him.
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