"I think the time has come when parties do not matter anymore. This is a problem affecting the nation," Dr Mahathir told reporters in a pitch for political adversaries to put aside their differences to act against Mr Najib, including members of the long-ruling United Malays National Organisation.

"I want everybody to act as citizens who are concerned with what is happening in this country," he said.

"As a citizen, it does not matter whether you are for the opposition or the government … now is the time to act as citizens, irrespective of party affiliations or loyalties."

Dr Mahathir was recruited last month to head an alliance in UMNO that was set-up to plot Mr Najib's ouster after months of scandal and controversy, including the prime minister's refusal to explain how almost $1 billion turned up in his private bank account in 2013 and what happened to millions of dollars that are still unaccounted for.

But this is the first time that 90-year-old Dr Mahathir, who led Malaysia for 22 years, has called for people to unite to rise up against Mr Najib, who has moved to fortify his position with powerful UMNO division chiefs who have benefited for years from money-politics and the party's largesse.

"The people support me quietly so Najib cannot hear. I need to make it clear to Najib that it's not me, it's the people," Dr Mahathir said.

"Lots of them are concerned about this kind of administration based on cash is king," he said.

Dr Mahathir has been one of the fiercest critics of British-educated Mr Najib, a close ally of successive Australian governments.

While his authority in UMNO has waned since he stepped down from office in 2003, Dr Mahathir remains a significant figure in the party that has ruled since Malaysia's independence from Britain.

He did not say how the people should rise up.

But his appeal could stoke moves for mass anti-Najib rallies across the country.

Mr Najib's government has stepped up pressure on Dr Mahathir in recent weeks as he has waged a relentless destabilisation campaign.

Last month, Dr Mahathir's son Mukhriz Mahathir was forced to resign of chief minister of the state of Kedah, when 12 division leaders of UMNO turned against him.

Analysts believe Mr Najib orchestrated the ouster.

On Wednesday, police questioned Dr Mahathir over comments he made in his blog alleging that Mr Najib was being protected by the country's attorney-general.

"I did not answer the policeman's questions on the article as I wish to answer in court during a hearing or trial," he said, adding he was prepared to go to jail.

"If you stick your neck out, you will be chopped. If that is going to be the end of my life – in jail, okay, that's it," he was quoted as saying by the Malaysiakini news website.

When Dr Mahathir was prime minister he ruthlessly pursued his former deputy prime minister and later opposition leader Anwar Ibrahim, who is serving a five-year jail sentence of what he says are politically motivated sodomy charges.

Asked if he foresaw ending up in the same jail as Mr Anwar, Dr Mahathir quipped: "Maybe we'll share the same cell."

Elections are not scheduled in Malaysia for two years, giving Mr Najib time to restore faith in his leadership.

But whether he can keep the support of the UMNO powerbrokers will depend on whether he can ride out the scandals and oversee the repairing of a sluggish economy.

Investigations are underway in the United States, Singapore, Hong Kong and Switzerland into Malaysia's sovereign wealth fund 1Malaysia Development Berhad, which Mr Najib set-up and oversees through an advisory committee.

The fund is struggling to re-pay billions of dollars in debt.

Investigations are also underway in France into alleged kick-back payments over Malaysia's purchase of French-Spanish submarines when Mr Najib was defence minister.

Mr Najib denies any wrongdoing and says the allegations are part of a campaign of smear that is being waged against him.