A four-party alliance driven by Mahathir and Anwar won the general election last week, ousting the ruling Barisan Nasional coalition for the first time in the history of the Southeast Asian nation.
Voters turned out en masse to oust Najib Razak after he was accused of involvement in plundering huge sums from Malaysian sovereign fund 1MDB in a sophisticated fraud that is now being investigated in several countries.
New Malaysian Prime Minister Mahathir Mohamad vowed on Sunday to review a controversial law against "fake news" hurriedly passed before the elections and seen as aimed at critics of his scandal-tainted predecessor.
In a huge political upset, former strongman Mahathir's opposition alliance broke the grip on power of the Barisan Nasional (BN) coalition, which had governed Malaysia uninterrupted since 1957.
Official results from the Election Commission of votes cast showed that Prime Minister Najib Razak's ruling coalition, Barisan Nasional (BN), had so far won 25 of parliament's 222 seats and Mahathir's alliance had won 16.
Malaysian authorities are investigating opposition leader Mahathir Mohamad under an anti-fake news law over claims that his plane was sabotaged in the run up to a general election next week, police said on Wednesday.
Malaysian ex-leader Mahathir Mohamad warned Friday elections next month will be the dirtiest in the country's history due to cheating by the "monster" prime minister, but insisted his opposition alliance has a fighting chance.
Najib Razak is widely expected to retain power, especially with a deep rift between Mahathir Mohamad's opposition alliance and the Pan-Malaysian Islamic Party (PAS).
Leaders from across Malaysia's political spectrum joined forces Friday to call for the removal of scandal-hit premier Najib Razak, in a sharp escalation of a festering corruption crisis.