Growing opposition to the Hong Kong government has plunged the financial hub into its biggest political crisis in decades and poses the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012.

Speaking at a conference in Kuala Lumpur, Mahathir, referring to Beijing, said Lam "has to obey the masters and at the same time she has to ask her conscience."

"I think the best thing is to resign," Mahathir said, according to a recording of comments, heard by Reuters, that he made when asked what advice he might give Lam.

Mahathir also said he expected China to take action against the protesters, drawing a parallel to student protests in Tiananmen Square in 1989.

"Yes, they allowed you to demonstrate and all that but eventually in a system that is authoritarian they will come back and do what they have to do," he said.

Mahathir, 94, is one of Asia's most experienced and outspoken leaders. He was prime minister for 22 years from 1981, only to come out of retirement last year to head a government after an opposition election victory.

The protests in Hong Kong began over a now-withdrawn extradition bill, which would have allowed people to be sent to mainland China for trial, and have intensified markedly since June. They have also evolved into wider calls for democracy, among other demands.

Demonstrators are angry at what they see as creeping interference by Beijing in their city's affairs despite a promise of autonomy in the "one country, two systems" formula under which Hong Kong returned to China in 1997.

China dismisses accusations it is meddling and has accused foreign governments, including the United States and Britain, of stirring up the anti-China sentiment.

REUTERS