Hong Kong riot police clashed Monday with pro-democracy protesters for a third straight day, as the city's leader warned the global financial hub was nearing a "very dangerous situation" and a rare strike caused transport chaos.

Clouds of tear gas billowed across multiple locations on Monday afternoon as the city buckled under a general strike, which protesters pushed to emphasise they still had broad public support following two months of increasingly violent unrest.

In a rare public appearance since the crisis began, chief executive Carrie Lam warned protesters she would not cave to their demands.

"They have seriously undermined Hong Kong's law and order and are pushing our city, the city that we all love and many of us helped to build, to the verge of a very dangerous situation," Lam said.

She later referenced chants by protesters for a "revolution", describing this as a challenge to the "one country, two systems" framework under which Hong Kong has been ruled since it returned from British to Chinese rule in 1997.

"I dare say they are trying to destroy Hong Kong," said Lam, who was appointed by a pro-Beijing committee.

China's cabinet-level State Council said it planned to hold a press conference on Tuesday.

Lam spoke on a day that saw widespread civil disobedience across the city. 

Activists descended on subway stations during morning rush hour, deliberately keeping open doors to stop trains departing and paralysing large parts of a network that millions of people use daily.

In the afternoon they held simultaneous rallies at seven locations and besieged multiple police stations, stretching the resources of a force that has become a lightning rod for public anger.

Tear gas was fired in four separate locations, making Monday's clashes the most widespread so far.