Leaders from across Malaysia's political spectrum joined forces Friday to call for the removal of scandal-hit premier Najib Razak, in a sharp escalation of a festering corruption crisis.

"We call upon all Malaysians, irrespective of race, religion, political situation, creed or parties, young and old, to join us in saving Malaysia from the government headed by Najib Razak," read a joint statement endorsed by heavyweights from the ruling party, opposition, and civil-society groups.

The historic alliance brought together previously bitter political foes and was led by 90-year-old former premier Mahathir Mohamad, who has spearheaded calls to remove Najib over allegations of corruption and misrule.

Speaking at a Kuala Lumpur press conference, Mahathir -- who quit the Najib-controlled ruling party earlier this week in protest -- said the disparate group shared "one goal."

"We must rid ourselves of Najib as prime minister," he said.

The call is the biggest challenge yet to Najib, voicing a growing sense of public disgust with his tenure.

 'Badly tarnished Malaysia'

 Najib, 62, has been under fire for a year over allegations that billions of dollars were stolen from a state firm he founded, and his own acceptance of a murky $681 million overseas payment.

Reports also have emerged of the luxurious lifestyles, lavish spending and jet-set travels of his family.

Even before the graft allegations emerged Najib had faced mounting criticism for muffling dissent, allowing multi-racial Malaysia's ethnic divisions to widen, and poor economic stewardship.

Najib denies wrongdoing, saying the corruption accusations are part of an unspecified political conspiracy.

He has counterattacked by curbing multiple investigations and purging his ruling United Malays National Organisation (UMNO) of critics, essentially shutting off any internal party challenges.

Whistle-blowers have been arrested, while media outlets reporting on the allegations have been muzzled, raising growing concern for rights and freedom of expression.

"Today Malaysia is badly tarnished," the joint statement said, calling for broad political reforms.

Those present included former deputy premier Muhyiddin Yassin, whom Najib sacked last year after he called for a probe into the funding mess.

They also included top leaders of Malaysia's opposition parties and of an electoral-reform movement that staged huge demonstrations last year over the scandals.

Critics allege that perhaps billion of dollars were skimmed from state investment firm 1Malaysia Development Berhad (1MDB) in complex overseas transactions.

1MDB was established in 2009 by Najib. It denies wrongdoing.

But US authorities are reportedly looking into 1MDB-related fund flows, while Swiss, British, Singaporean and Hong Kong authorities have acknowledged scrutinising the affair.

Swiss authorities said recently they believe up to $4 billion may have been stolen from Malaysian state firms and that they were investigating possible fraud and money-laundering.

Najib at first hotly denied accepting the mysterious $681 million payment put in his personal bank accounts in 2013.

But the government now claims it was a gift from the Saudi royal family, most of which was returned.

The Saudis are yet to officially confirm that claim, which is broadly ridiculed in Malaysia as far-fetched.

An investigative report this week by the Wall Street Journal said it found fresh clues that the money originated from 1MDB, and no Saudi involvement.

Najib's office did not immediately offer comment.

 'Momentum is building'

 "The momentum is building against Najib and is stronger than before. It can possibly snowball into bigger pressure, but the resistance from the government and UMNO will be strong as well," said Wan Saiful Wan Jan, head of the Malaysian think tank IDEAS.

Najib has weathered the pressure by rallying loyal UMNO leaders and using the party's tight grip on the country to secure his position.

The new movement announced no plans for public protests. Its future strategy was not entirely clear.

The escalating crisis has brought warnings that fears of political instability could impact foreign investment just as the Malaysian economy's reliance on energy exports is being strained by low oil prices. 

UMNO has dominated Malaysia since independence in 1957 by championing the rights of Muslim Malays, Malaysia's majority ethnic group.

But voters have increasingly rebelled against its divisive racial politics, authoritarian tactics, corruption, and an electoral system seen as pro-UMNO.