The leader of Spain's opposition Socialist party called on Sunday for Prime Minister Mariano Rajoy to resign amid a damaging corruption scandal.
"Rajoy should give up his role as the head of government (because) he cannot tackle the very difficult situation confronting Spain," Alfredo Perez Rubalcaba told reporters.
Spain's centre-left newspaper El Pais published account ledgers purportedly showing that donations were channelled into secret payments to Rajoy and other top party officials.
Rajoy has dismissed the ledgers as false and has vehemently denies receiving any payments.
The prime minister "cannot lead the country in this delicate time," Rubalcaba said.
"His presence (at the head of the government) is not going to help resolve this political crisis," the opposition leader said.
The scandal has infuriated Spaniards, millions of whom are still struggling to find work in an ongoing recession with the highest jobless rate, at 26 percent, since the return of democracy in 1975.
Many feel they have been forced to bear the brunt of an economic crisis triggered by the collapse of a construction boom, which they blame on corrupt politicians and unscrupulous banks.
Since the scandal broke on Thursday, hundreds of angry demonstrators have mounted daily protests outside Rajoy's Popular Party (PP) offices, waving envelopes symbolising the envelopes stuffed with cash that were allegedly distributed to party members.
At Sunday's protest around 100 people hemmed in by barricades and police vans shouted "Resign!", "Delinquents!" and "This Isn't a Party, It's a Mafia!"
An online petition to oust Rajoy had gathered 766,000 signatures by midday on Sunday.
In an emergency meeting with PP members on Saturday, Rajoy fought off the allegations and vowed not to resign.
"I have never received nor distributed undeclared money," he said.
"I commit myself personally and all of my party to maximum transparency," he added, promising to publish full details of his earnings and assets.
According to Rubalcaba, Spain needs "a strong government that inspires confidence and the government does not do that, starting with its leader."
Rajoy had "added a problem, a moral crisis," he said.
The published ledgers show details of donations, mostly from construction companies, that were then allegedly channelled into a fund benefiting Rajoy and other top officials.
El Pais cited ledgers kept by former party treasurer Luis Barcenas, apparently showing payments including 25,200 euros ($34,400) a year to Rajoy between 1997 and 2008.
Had the payments been declared, they would be legal, the paper said.
Barcenas, who is already under investigation over reports that he had stashed millions of euros in Swiss bank accounts, denied the allegations, as did most of the top officials named in the report, who include the party's current secretary general Maria Dolores de Cospedal.
She rejected the allegations on behalf of the party, insisting that its accounts were "transparent".
A poll published in El Pais on Sunday showed that the PP's popularity rating had dropped to 23.9 percent, the lowest since it came to power in 2011, and only just higher than the Socialists on 23.5 percent.
Spain's authorities have meanwhile sought to play down the affair, with Deputy Prime Minister Soraya Saenz de Santamaria saying the government enjoyed "great stability".
Rajoy swept to power in a 2011 election when voters angry at the economic crisis kicked out the Socialists. He defied speculation that the country would need a financial bailout in 2012, only for the political scandal to erupt in the new year.