Describing the coalition as the start of a "national realignment" movement, Mahathir led a group of more than 50 politicians yesterday to sign the Citizens' Declaration, seeking the removal of Najib.
The unprecedented show of solidarity cuts across the political divide and many civil society leaders and opposition politicians have also joined their former rival Mahathir, who was Malaysia's premier for 22 years from 1981 to 2003, and largely credited for the country's fast growing economy.
It has attracted the unlikeliest of allies including oppositionDAP party adviser Lim Kit Siang, who was detained under the Internal Security Act during Mahathir's tenure, PKR party deputy president Azmin Ali and Amanah president Mohamed Sabu.
Earlier on Monday, Mahathir, 90, had quit the ruling Umno party, saying he could no longer stay in Umno because the party had turned into an organisation dedicated to supporting the scandal-plagued Najib.
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"It's not about joining the opposition or any group. It's about citizens joining together to show support," said Mahathir, reading from a memorandum agreed by 58 signatories.
Najib has faced sustained pressure to resign since last year over the scandal surrounding state fund 1Malaysia Development Berhad (1MDB), amid allegations that funds worth USD 680 million from 1MDB had been channelled into his private accounts.
Najib maintained that he has not used the funds for personal gain, and has since been cleared of any criminal offence or corruption.