Police fired tear gas in at least 11 areas across the city, battling to disperse crowds of demonstrators in the most widespread unrest seen during the two months of increasingly violent protests.

The demonstratons 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.

Earlier Monday, Hong Kong buckled under the strike urged by protesters who wanted to emphasise they still had broad public support.

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 referred to 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.

During Monday's widespread unrest, activists descended on subway stations at morning rush hour, deliberately keeping open doors to stop trains departing and paralysing large parts of a network that millions of people use daily.

They later 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.

The most sustained clashes occurred outside the city's parliament where protesters used giant slingshots to launch bricks and the working class area of Wong Tai Sin, with tear gas fired throughout the day.

In the evening, a group of men armed with long sticks, clashed with protesters in North Point, a neighbourhood where many Chinese migrants live and is known for its strong Beijing ties. They chased and beat protesters, who threw traffic cones and barricades in return.

- Protesters unbowed -

In a briefing that highlighted the longevity of the protests, police said they had fired more than 1,000 rounds of tear gas and 160 rubber bullets since rallies began on June 9, with 420 people arrested and 139 officers injured so far.

But the protesters remain unbowed.

"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.

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

More than 160 flights at the city's airport were listed as cancelled on Monday. Many were with Cathay Pacific, Hong Kong's flag carrier.

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

Some major roads were also blocked and 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.

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

Another showed a taxi ramming protesters who hurled projectiles.

But while some locals 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.

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