Mahathir said in a video posted online that Malaysia is “facing a state of panic’’.
Najib, who is eyeing an early election in 2017, faces outrage over his involvement in a multi-billion-dollar financial scandal at state fund 1Malaysia Development Berhad (1MDB), and has used harsh measures to silence critics.
However election reform group Bersih, the organiser of Saturday’s rally, has demanded Najib resign.
Mahathir, who quit Najib’s ruling United Malays National Organisation (UMNO) in February, said in a video posted online that Malaysia is “facing a state of panic’’.
“I hope all Malaysians will join this demonstration by Bersih which is aimed at finding a way to fix this country,’’ Mahathir said.
Malaysia’s longest serving Prime Minister formed a new political group this year in his campaign to oust former protege Najib.
Meanwhile, concerns were growing that Bersih supporters and pro-government groups could clash at the rally, with the group’s chairwoman having received anonymous death threats.
“We cannot stop because if we stop and we do not protest, then we cannot have any say in any legislation, policies or laws,’’ the chairwoman, Maria Chin Abdullah, who led a similar rally in 2015, said.
Ethnic Indian and Chinese minorities formed the bulk of the 200,000 protesters at the time.
A six-week campaign by Bersih ahead of the rally has been marred by violent confrontations between the group and a pro-state group called “Red Shirts”.
Red Shirts leader Jamal Yunos, an UMNO member, has warned of a repeat of racial riots in 1969, when clashes between Malays and ethnic Chinese killed hundreds.
Mainly Muslim ethnic Malays form about 60 per cent of Malaysia’s population of 30 million, while ethnic Chinese and Indians account for about 32 per cent.