Mahathir's focus is also to tackle corruption scandals that have dogged Najib's administration.
Earlier this week, Najib's long-ruling Barisan Nasional coalition suffered a shock electoral defeat.
Speaking at a news conference, Dr Mahathir said he would announce a 10-member cabinet on Saturday, including ministers for finance, defence and home affairs.
Police has stepped up security at the entrance to the private residence of former Prime Minister Datuk Seri Najib Razak at Jalan Langgak Duta.
The Immigration Department, which earlier denied that Najib and his wife were barred from leaving, says in a brief statement Saturday the duo have "just been blacklisted from leaving the country".
US Set to Open Jerusalem Embassy Opposed Across Middle East
Marchers have thrown stones and burned tires at the fence and flown flaming kites over it to try to set Israeli fields on fire. Israeli Prime Minister Benjamin Netanyahu called on "all countries to join the USA in moving their embassies to Jerusalem ".
Mahathir and Anwar reconciled in recent years in a bid to oust Najib, an extraordinary twist even by the head-spinning standards of Malaysian politics.
A few minutes later Najib posted on Twitter that he respects the direction by the Immigration Department and would remain in the country and has cancelled his overseas trip.
"It is true that I prevented Najib from leaving the country" Mahathir told reporters on Saturday.
"It's been cleared, there's been no wrongdoing - I stand by it, " Najib said in an interview last month.
Google to roll out with new feature 'Take a Break' for YouTubers
The new feature is now available on YouTube Android app version 13.17+ and comes in line with Google's attempt with Android P. One will disable all notification sounds and vibrations in the YouTube program for a fixed amount of time.
Malaysia's new finance minister is seen as a safe pair of hands as he takes on the huge task of bringing down debt while scrapping a consumption tax that was key to Prime Minister Mahathir Mohamad's unexpected election win last week.
"It is a very complex thing because it involves a lot of people, it involves a lot of decisions made, and the money you know has to be investigated as to the money laundering", he said.
The National Front, which has ruled Malaysia since independence from Britain, obtained only 79 of the 222 parliamentary seats, losing power to Mahathir Mohamad's four-party alliance amid public anger over the 1MDB scandal and rising cost of living.
Possible return of Anwar Ibrahim: Mahatir said after his victory over Najib that the country's monarch had agreed to pardon jailed opposition politician Anwar Ibrahim, who has been serving a jail sentence since 2015 on "sodomy" charges that Anwar says were politically motivated.
U.S. Department of Justice documents allege that $681 million from 1MDB was transferred to the personal account of a person identified as Malaysian Official One, which U.S. and Malaysian sources have confirmed was Najib.
English Premier League match report Southampton v Manchester City 13 May 2018
Chelsea is off to the Europa League, and in poor form ahead of next week's FA Cup Final against Manchester United . City also added another landmark by winning the league by a record 19 points from local rivals Manchester United .
He denied any wrongdoing, even as US authorities alleged that over $4.5 billion was stolen from the fund in a fraud orchestrated by a financier known to be close to Najib and his family.