In an interview with AFP a day after he walked free from custody -- where he had languished since 2015 on what supporters say were trumped-up charges levelled at Najib's behest -- Anwar also said he would be back in parliament very soon.
"Give me a few months, I should be back as an MP. It is the correct thing to do," he said.
Anwar's release on Wednesday capped an astonishing week in Malaysian politics that saw Najib's long-ruling Barisan Nasional coalition booted from power in a revolution at the ballot box.
It also imperilled Najib, who Anwar said now faces an "arduous" legal battle -- without the protection of high office.
Accused of overseeing the plundering of state fund 1MDB, Najib's position has looked increasingly precarious. Police carried out extensive searches of his home overnight.
Anwar predicted that Najib would likely take up the spot he had just vacated in prison. "He will certainly be charged," he said.
Anwar, 70, declined to say how he thought Najib's case would play out, as it depended on "how he can defend himself in court".
But Anwar added: "It will be very difficult for him to escape (going to) prison." Najib and 1MDB deny any wrongdoing.
Mahathir, who came out of retirement to lead the revolt against Najib, has signalled he will hand power to Anwar within two years.
The younger man's return to politics has been the subject of much speculation and he confirmed he would be seeking to return as an MP soon -- but would not seek a position on the front benches just yet.
"I am not intending to join the cabinet," he said he had told Mahathir.
Anwar enjoyed a meteoric rise in the now-ousted Barisan Nasional and was heir-apparent to the premiership until he fell out with Mahathir in the 1990s and was sacked.
He was subsequently jailed on widely-criticised charges of sodomy and corruption, in a case that gripped the country and triggered unprecedented mass protests.
Anwar emerged from prison to unite a previously hapless opposition and lead them to historic electoral gains, before being thrown in jail again.
But in a dramatic turnaround, Mahathir reconciled with Anwar when he was behind bars serving his second jail term, and joined forces with the opposition.
Anwar conceded he had had "ups and downs" with Mahathir but added their relationship was now "very cordial, very friendly" and signalled he would let Mahathir take charge of running the country for now.
"I've told Mahathir that he should feel free to continue to manage the affairs of this country. I will support him," he said.
But Anwar added that he would "express my views strongly" if he felt it necessary.
Mahathir ruled with an iron first during his first stint in office, and was accused of throwing political opponents in jail and undermining the country's democratic institutions.
But Anwar said the elderly politician had pledged to make amends and is now committed to reform, adding: "I think we should welcome that." Wong Chin Huat, a political scientist from think-tank the Penang Institute, said he believed the rivals turned allies could work together for a peaceful transition of power.
"The circumstances that the country is in now are very clear," he told AFP. "There is a managed transition that has been agreed upon and accepted by all parties since well before the elections.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)