MADRID (AFP) - Spain's Prime Minister Mariano Rajoy on Saturday denied allegations that he received undeclared payments from his ruling party as he fought off a major corruption scandal in the face of angry street protests.

Rajoy vowed not to resign despite the publication of documents purportedly showing secret payments to him and other top party officials, branding the damaging reports "harassment".

He promised to publish full details of his income and assets, speaking at an emergency meeting of his conservative Popular Party as angry demonstrators outside called for him to step down.

"I have never received nor distributed undeclared money," he said, adding that he would publish online his "statements of income, patrimony and any information necessary" to refute the allegations.

"I commit myself personally and all of my party to maximum transparency," he said.

Mr Rajoy, 57, was speaking out for the first time since being named in the scandal which struck at a tense time as the government imposes tough spending cuts on Spaniards in a recession.

His speech did nothing to calm street protests, which erupted for a third day running near the party's headquarters in Madrid.

Hundreds of people rallied behind police barricades there during and after his address, yelling for him and other party leaders to resign.

"They are thieves and liars," said one demonstrator, Azahara Montes, 31.

"They are imposing tax rises on us while they live like kings. Rajoy can't deny anything. I believe there is proof and it will keep coming out." Leading centre-left newspaper El Pais on Thursday published account ledgers purportedly showing that donations were channelled into secret payments to Mr Rajoy and other top party officials.

The newspaper said the alleged fund was made up of donations, mostly from construction companies.

The allegations - following numerous corruption cases implicating lower-ranking local politicians - fuelled anger among Spaniards suffering in an economic crisis that has thrown millions out of work.

Protesters say ordinary Spaniards are being made to pay for the fallout from the collapse of a construction boom that many blame on corrupt politicians and unscrupulous banks.

An online petition at change.org calling for Mr Rajoy to resign, launched on Thursday, had gathered more than 700,000 signatures by Saturday evening.

"We must not allow Spaniards, of whom we are demanding sacrifices, to think that we do not observe the strictest ethical rigour," Mr Rajoy said.

On Thursday El Pais cited ledgers kept by former party treasurer Luis Barcenas, apparently showing payments including 25,200 euros (S$42,650) a year to Rajoy between 1997 and 2008.

Barcenas was already under investigation in connection with a separate corruption case, with reports that he had millions of euros in a Swiss bank account.

Mr Rajoy said that case had nothing to do with the party and that it had never had foreign bank accounts.

"I did not enter politics to make money," he said, adding that he made more money in his previous profession as a land registrar. "For me, money is not the most important thing in life." Deputy Prime Minister Soraya Saenz de Santamaria insisted on Friday that Rajoy's government was not destabilised by the party scandal.

But she said that "undoubtedly, the institutions of Spain need to be strengthened," with numerous corruption cases pending.

Even the royal family has been hit by scandal, with King Juan Carlos's son-in-law Inaki Urdangarin being investigated for suspected embezzlement.