In an interview with AFP, the 91-year-old former prime minister excoriated Najib over the sensational graft allegations swirling around the 1MDB sovereign wealth fund.
The man who strove to subdue Malaysia's opposition parties during his time in office from 1981-2003 said he could now work with them to topple Najib, including the party of his jailed former deputy Anwar Ibrahim.
"He (Najib) is destroying this country... He is bringing in racism... Very, very serious crime has been committed," Mahathir said yesterday at his office at Putrajaya south of the capital Kuala Lumpur.
Under mounting pressure over the allegations, the prime minister last year abruptly shut down Malaysian investigations, fired the attorney general and purged ruling party critics including his deputy Muhyiddin Yassin.
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Najib, 63, and 1MDB have denied any wrongdoing.
Mahathir was once Najib's mentor but has since come out of political retirement to attack him over the debt-laden 1MDB. He has formed a political party in hopes of toppling the ruling United Malays National Organisation (UMNO).
Anwar was heir apparent to Mahathir until he was sacked in 1998 over their political differences. Anwar was subsequently charged and jailed for sodomy, but the pair appear to see eye-to-eye over the 1MDB scandal.
Last month tens of thousands of Malaysians flooded Kuala Lumpur to express anger over the graft allegations.
At the rally Mahathir said Malaysia was "controlled by thieves" and called for a sustained push to topple the prime minister. Critics accuse Mahathir of hypocrisy, saying he also tolerated corruption and repressed dissent in his day.