Former Malaysian leader Mahathir Mohamad joined with political foes Friday in issuing a declaration signed by 58 public figures urging Prime Minister Najib Razak to resign over corruption allegations.

Najib is resisting growing pressure to step down after documents leaked last July showed more than $700 million was deposited in his private bank accounts. The attorney general decided in January not to prosecute Najib, saying most of the money was a personal donation from Saudi Arabia's royal family.

News reports since then have said the total may exceed $1 billion.

Mahathir said the country's reputation has been tarnished, with Najib unable to quash accusations the money came from indebted state investment fund 1MDB, which he founded in 2009.

"We must rid ourselves of Najib" as prime minister, Mahathir said. "If he's allowed to go on, the damage will be worse and worse."

Mahathir was prime minister for 22 years before stepping down in 2003. He remains an influential political figure and since last year has led calls for Najib to resign.

The declaration comes four days after Mahathir quit the ruling Malay party after accusing Najib of hijacking it to protect his own interests. In addition to Mahathir and key opposition figures, prominent civil society leaders and several ruling party lawmakers, including former Deputy Premier Muhyiddin Yassin, also signed the statement.

The prime minister's office, in an immediate response, said "Mahathir and his former enemies have demonstrated the depth of their political opportunism and desperation." It said Mahathir must follow the democratic process and wait for the next general election, due in 2018, if he wants to change the government.

The declaration is a show of unity by disparate forces who hope the call for Najib to resign will gather momentum nationally. They face a tough challenge as Najib has kept an iron grip since the scandal began by replacing critics in the Cabinet and ruling party with loyalists and cracking down on the media.

Najib also replaced the former attorney-general who had been investigating him with a loyalist, who last month ordered the anti-corruption agency to close its investigation of the prime minister. The agency is appealing the decision.

The Wall Street Journal reported Tuesday that investigations into 1MDB have revealed that a total of more than $1 billion was routed into Najib's bank accounts — several hundred million dollars more than had been reported before — and that investigators believe most came from 1MDB through a complex web of transactions in several countries.

1MDB is mired in 42 billion ringgit ($10 billion) in debt and has been selling its assets to clear its books. Both Najib and 1MDB have repeatedly denied any wrongdoing.