0417 GMT November 19, 2019
Immigration authorities issued a travel ban on Najib and his wife Rosmah Mansor just minutes after the ousted prime minister said they were leaving on a weeklong trip overseas to rest after his thumping electoral defeat, Reuters reported.
"It is true that I prevented Najib from leaving the country," Mahathir said at a news conference, adding that doing so had averted extradition problems later.
"There is sufficient evidence that an investigation into certain things ... done by the former prime minister has to be done and, if necessary, the rule of law will apply," he said.
During a day of fast-moving events, Mahathir also named his first few cabinet ministers, including Lim Guan Eng, a former banker and qualified chartered accountant, as finance minister. Lim is the chief minister of Penang state, but is largely unknown in international financial circles.
It is also only the second time since Malaysia became independent six decades ago that the post has gone to a member of the ethnic Chinese minority.
Mahathir also named a defense minister and a home or interior minister, but said other appointments would be made later. Mahathir and Deputy Prime Minister Wan Azizah Wan Ismail, the wife of his jailed ally Anwar Ibrahim, make up the rest of the cabinet.
Zeti Akhthar Aziz, who was internationally lauded during a 16-year stint as central bank governor, and billionaire tycoon Robert Kuok were among those named to a special team that will advise the government on economic and financial matters for the next 100 days.
Mahathir has been prime minister of the Muslim Malay-majority nation earlier, for 22 years, governing in a tough, pugnacious style.
He said the attorney general, who had cleared Najib of wrongdoing in the graft scandal at state fund 1Malaysia Development Berhad (1MDB), had been sacked.
Apandi Ali, the attorney general, declined to comment.
As Najib came under increased pressure, the world seemed to be opening up for Anwar, who, from jail and a hospital bed, combined with Mahathir to hand out the defeat to the administration alliance.
Mahathir has said the king has indicated to him that a royal pardon for Anwar would be announced soon.
After the ban on his travel was announced, Najib said in a Twitter message that he would respect the decision and would remain in the country.
Questions about his whereabouts were answered when he appeared at a meeting of his United Malays National Organization (UMNO) to announce that he was stepping down as the party's president and as chairman of the Barisan Nasional, the alliance dominated by UMNO that has ruled Malaysia for six decades.
Najib lost the election at least partly because of popular disgust over the 1MDB scandal.
News broke in 2015 that about $700 million allegedly stolen from 1MDB had made its way into his personal bank accounts.