Hong Kong’s leader has warned that the city is on “the verge of a very dangerous situation” after protesters blocked roads and paralysed train services during morning rush hour.
More than 14,000 people from 20 sectors vowed to join a city-wide strike on Monday, its organisers said.
Civil servants, who are ordered to be politically neutral, have reportedly agreed to take part.
At the airport, more than 200 flights were cancelled amid disruption fears.
The protests have now continued for nine consecutive weekends. After violent clashes on Saturday and Sunday, police again fired tear gas at protesters on Monday.
Police said they had fired 1,000 tear gas canisters and 160 rubber bullets, and had made 420 arrests, since 9 June, when the protests began.
In her first media address in two weeks, the city’s leader Carrie Lam said the protesters’ actions had challenged the principle of “one country, two systems” and were threatening prosperity and stability in Hong Kong.
She also accused activists of using a controversial extradition bill, which triggered the protests in June, as a cover for their real goals.
“We continue to allow these violent protesters to make use of the [extradition] bill to conceal their ulterior motives,” she said. “Those ulterior motives are going to destroy Hong Kong.”
Delays have affected the Mass Transit Railway (MTR) as scuffles broke out between commuters and activists, who held open the doors to stop trains leaving. Several MTR lines have now been suspended.
One video circulating on Twitter reportedly showed a car in the district of Yuen Long forcefully hitting a barricade set up by protesters, injuring one person.
It is not yet clear how many have joined the planned city-wide strike, but several shops and businesses were closed – including international fashion retailers like Topshop and Zara.
As well as the strike, activists had scheduled rallies in seven different parts of Hong Kong on Monday.
Most of the cancelled flights are with local carriers Cathay Pacific and Hong Kong Airlines.
Hong Kong airport, one of the busiest in the world, said travellers should check its website and seek updates directly from the airlines.
“The Airport Authority advises passengers to check with their airlines for the latest flight information, and to proceed to the airport only when their seats and flight time have been confirmed,” the airport said in a statement.
What’s behind the Hong Kong protests?
The protests were initially sparked by a controversial bill that would allow China to extradite suspects from Hong Kong to the mainland.
Critics said it would undermine the territory’s judicial independence and could be used to target those who spoke out against the Chinese government.
Although the bill has now been suspended, demonstrators want it fully withdrawn.
Their demands have broadened to include Ms Lam’s resignation, and the dropping of riot charges linked to the protests.
Hong Kong – a former British colony – is part of China, but enjoys unique freedoms not available on the mainland.
Last week, more than 40 activists appeared in court charged with rioting. If convicted, they could be jailed for up to 10 years.
China’s People’s Liberation Army (PLA) has so far stayed out of the dispute, but China’s top policy office in Hong Kong has previously condemned the protests, calling them “horrendous incidents” that have caused “serious damage to the rule of law”.
Unease rose last week when China’s army in Hong Kong posted a video on Chinese social media network Weibo showing soldiers conducting anti-riot drills.