In "an initial stage, maybe lasting one or two years, I will be the prime minister", Mahathir said, speaking by live video link from Kuala Lumpur to a Wall Street Journal CEO conference in Tokyo on Tuesday.
Anwar's wife Wan Azizah Wan Ismail, who is the new deputy premier, has said there was no rush for Anwar to take over. She said the full pardon for her father, who is now in a hospital recovering from shoulder surgery, was sought due to a "miscarriage of justice". Najib has said the deposit was a donation by an unnamed member of the Saudi royal family which had been largely returned.
Malaysia's king has agreed to pardon him, paving the way for his return to politics.
Anwar is prime minister-in-waiting but unlikely to take over quickly from Mahathir Mohamad, who after leading Anwar's alliance in the election campaign has become the world's oldest leader at 92.
"So, we expect (the release) will be at noon tomorrow before a gathering in the afternoon in Petaling Jaya", he told reporters after visiting Anwar at the hospital today.
Just three years ago, Anwar's career seemed finished, once again, when he was jailed on charges of sodomizing a young male aide - the second time he had faced such charges.
"For the time being, I will run the country", Mahathir said, adding that he will lead for "one or two years". "There will be no special powers given except as are given to ministers or deputy ministers or deputy prime ministers", Mahathir said.
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But as she posed for the shutterbugs for pictures, she removed her stilettos and walked barefoot on the red carpet. The Twillight star joins Lupita Nyong'o, Marion Cotillard, Penelope Cruz and Bingbing Fan.
The corruption-plagued former government of now-disgraced Najib Razak was keenly aware of this, and the latest sodomy charge dogged Anwar through a lengthy trial that culminated in his imprisonment in 2015.
Jack Seng, another supporter wearing a shirt with a logo of Anwar behind bars, said he thought Mahathir and Anwar would resolve any tensions between them. "We will be able to charge him".
"I think we need to let Mahathir do his work to save Malaysia and to get back its system of government".
The charismatic Anwar was a rising political star in the 1990s under Mahathir, heading various ministries and earning praise overseas as a reformer.
The pardon allows Anwar to re-enter active politics immediately, but it was not clear what his role in government would be. He led an opposition alliance to major gains in the last two elections, winning the popular vote in 2013.
"I have given my assurance, I am here as a concerned citizen to give complete support to manage the country on the understanding that we are committed to the reform agenda, beginning with the judiciary, media and the entire apparatus", Anwar said in a news conference at his home after he was freed from custody. "Authoritarian leaders always believe the best way to deal with dissidents is to jail them, but throughout history, it has always backfired". It regrouped as the Alliance of Hope, but it was Mahathir's political comeback and his embrace by Anwar's party that helped seal the opposition's victory. The fund was set up by Najib in 2009 to promote economic development but accumulated billions in debt.
He had barred Najib from leaving Malaysia and pledged to reopen a corruption probe into state fund 1MDB.