Clouds of tear gas billowed across the working-class district of Wong Tai Sin on Monday afternoon as the city buckled under rare 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.

She 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 seven simultaneous rallies, stretching the resources of police who fired tear gas in at least two locations.

The most sustained volleys were fired in Wong Tai Sin, known for its famous Taoist temple, where local residents and protesters had previously fought with police over the weekend.

"Support for the political strike today seems strong and it has been bolstered further by the escalating violence between the police and protesters," political analyst Dixon Wong told AFP.

- Flights cancelled, shops closed -

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The strike -- a rare occurrence in a freewheeling finance hub where unions traditionally have little sway -- hit the vital aviation sector.

More than 160 flights at the city's airport -- one of the world's busiest -- were listed as cancelled on Monday afternoon.

Many disrupted flights were with Cathay Pacific.

The carrier did not give a reason for the cancellations, but its flight attendants union confirmed some of its members had walked out.

"Over the last 50 days, the government has been ignoring the demands of the people and using only police force to try to suppress voices, causing countless Hong Kong people to despair," the union said in a statement on its Facebook page.

Some key roads were also blocked, causing gridlock.

Many shops across the city were shuttered, including big-name fashion outlets in the central commercial district.

The strike led to some scuffles between angry commuters and protesters at crowded subway stations, with videos circulating across social media highlighting tensions throughout the city.

One video, verified by AFP, showed a car smashing its way through a protester roadblock in the northern town of Yuen Long.

But while some commuters were angered by the disruptions, others said they supported the action.

"As long as the government doesn't respond then for sure the movement will escalate," a civil servant, who gave his surname as Leung, told AFP as he tried to make his way to work.

- China warnings -

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The past fortnight has seen a surge in violence from both sides, with police repeatedly firing rubber bullets and tear gas to disperse increasingly hostile projectile-throwing crowds.

A week ago a group of men suspected to be linked to triads -- Hong Kong's notorious gangsters -- also attacked demonstrators, putting 45 people in hospital.

Dozens of protesters have been charged with rioting, which carries a jail term of 10 years.

The protests were triggered by opposition to a planned law that would have allowed extraditions to mainland China, but have evolved into a wider movement for democratic reform and a halt to eroding freedoms.

Under the terms of the 1997 handover deal with Britain, Hong Kong has rights and liberties unseen on the Chinese mainland, including an independent judiciary and freedom of speech.

But many say those rights are being curtailed.

Chinese authorities have in public largely left Hong Kong's government to deal with the crisis, although they have begun to issue their own warnings.

China's military last week described the unrest as "intolerable" and released a propaganda video of a drill in which troops quash a protest.