Leaders of Malaysia's Barisan Nasional party put their weight behind embattled Prime Minister Najib Razak, saying any bid to remove him must follow democratic process and legal ethics.

Deputy Prime Minister Ahmad Zahid Hamidi said the move against the prime minister was ethically wrong as he was elected in a democratic process, trying to deflect fire from the 'citizens' declaration' against Najib.

"Any bid to remove Najib without following the same democratic process is against legal ethics," Ahmad said, according to The Star newspaper.

Former prime minister Mahathir Mohamad, a long-time critic of the tainted prime minister, became the rallying point of anti-Najib momentum after he resigned from the ruling United National Malay Organisation (UNMO).

"It's not about joining the opposition or any group. It's about citizens joining together to show support," 58 prominent leaders said in a signed memorandum seeking Najib's removal.

Najib is reeling from allegations that he diverted hundreds of millions of dollars from the state-owned company into his personal account in the run up to the 2013 election.

More than $1 billion was deposited into Najib's account between 2011 and 2013, the Wall Street Journal reported last week, citing people familiar with the ongoing investigations.

In a surprise move, prominent opposition figure and Mahathir's arch rival Anwar Ibrahim offered support to the former prime minister.

"We must together chart a new way forward to save our beloved nation," Anwar said in a statement released from the jail.

In the face of daunting opposition, several of Najib's ministers expressed vocal support for the corruption-tainted prime minister.

The signing of the declaration was a charade, Communications and Multimedia Minister Salleh Said Keruak said, hitting out at the critics.

"They realise their move to oust Najib has failed and this is just a face-saving exercise. It is a sideshow to give an impression that the effort has not ended when in fact it ended months ago," he said.

Agriculture and Agro-Based Industry Minister Ahmad Shabery Cheek said Anwar's support for Mahathir was a double-edged sword.

"By supporting Dr Mahathir, he (Anwar) admits to whatever allegations Dr Mahathir made against him in 1998," the minister said.