Malaysian Prime Minister Mahathir Mohamad turned up late Monday in a show of support for his designated successor and former deputy Anwar Ibrahim for the latter's bid to return to the parliament through a by-election.

Breaking his decades-long practice not to campaign for by-election, Mahathir urged Malaysians to support Anwar at a large campaign gathering ahead of the polling day on Saturday.

"I ask the voters to lend their support to the government and to Anwar in this by-election," he told the crowd.

The 93-year old prime minister said being able to put aside past differences was key to the Pakatan Harapan's surprising electoral victory in the general election in May.

"People know that me and Anwar have not been so good (with each other). We must not recall past animosity and if we do we would not have been able to unite," he said.

For his part, Anwar lavished praises on the prime minister, calling him "the best man to lead Malaysia."

The reconciliation between Mahathir and Anwar was essential for Pakatan's electoral victory. Mahathir has promised to hand over the premiership to Anwar, but the relationship between the two remains under close scrutiny given their fall out two decades ago, when Anwar was sacked by Mahathir from the post of deputy prime minister.

The by-election was triggered by the resignation of the previous Member of Parliament for the Port Dickson constituency, paving the way for Anwar to return to the parliament.

Anwar was granted a royal pardon on his sodomy conviction following Pakatan Harapan's victory. He has since won the presidency of his People's Justice Party party unchallenged.

The by-election will see a multi-corner fight with Anwar facing one opponent from the opposition Pan Malaysian Islamic party and several independents, including his former aide who claims to have been sodomized by him that led to the his imprisonment in 2015.

Anwar is widely expected to win but voter fatigue and the diluting effect of so many candidates is expected to sap his majority.