Spanish Prime Minister Mariano Rajoy said he will address allegations that his party received illegal payments from party donors.
The scandal began after the Spanish newspaper El Pais published what it said were secret accounts kept by the ruling People's Party.
The handwritten documents were said to reveal 11 years of payments to Rajoy and other leaders, said Reuters.
Other documents show off-the-books payments to the party lasting over two decades, mainly from property developers.
Spain's current financial crisis is said to be largely the fault of overdevelopment and real estate speculation along its Mediterranean coast.
Rajoy's party has denied the allegations.
"The People's Party has only one set of accounts and it is clean, transparent and submitted to the official accounting authority," said the party's secretary general, Maria Dolores de Cospedal, according to the Guardian.
"We have absolutely nothing to hide."
Reuters reported that the documents show the prime minister was paid approximately $34,000 per year for over a decade through a slush fund.
The news agency pointed out that Rajoy's staid, technocratic image has been shaken by the allegations.
On Thursday, protestors gathered at the party's headquarters in Madrid calling for leaders to resign.