- Video report by ITV News Correspondent Geraint Vincent
One of the world's busiest airports cancelled all flights after thousands of Hong Kong pro-democracy protesters crowded into the main terminal.
Hong Kong International Airport said in a statement that Monday's demonstration "seriously disrupted" airport operations.
Hong Kong has experienced more than two months of mass protests calling for democratic reforms and an independent inquiry into police conduct, with both the protesters and police adopting ever-more extreme tactics.
In Beijing, the Cabinet's Hong Kong and Macao Affairs Office issued a statement saying the situation in Hong Kong was "beginning to show the sprouts of terrorism" and constituted an "existential threat" to the population of Hong Kong.
"One must take resolute action toward this violent criminality, showing no leniency or mercy," said the statement, attributed to spokesman Yang Guang.
"Hong Kong has reached an inflection point where all those who are concerned about Hong Kong's future must say 'no,' to law breakers and 'no' to those engaged in violence."
Only flights that had already begun boarding or those cleared for landing were allowed to use runways at the airport.
"All other flights have been cancelled for the rest of today," the airport statement said.
A massive traffic jam soon formed on the highway leading back to Hong Kong's city centre.
“All other flights have been cancelled for the rest of today,” the airport said in a statement.
Some protesters were seen walking toward the airport amid the stifling heat.
Earlier on Monday, Hong Kong police showed off water cannons that could be deployed in the case of future demonstrations, a development which Amnesty International has warned could lead to serious injuries if misused within the densely-populated city's confined spaces.
Legislators and journalists were invited to witness the display of extreme crowd control tactics, which came after a weekend of protests at the airport and on the streets of one of the city's main shopping districts.
During the weekend protests, website Hong Kong Free Press showed footage of one arrest that appeared to include officers in plain clothes pinning a demonstrator pressed to the ground.
The young man, who said his name was Chow Ka-lok and asked for a lawyer, was shown with a bleeding head wound and said he had a broken tooth.
Police have also reported injuries among their ranks, including eye irritation from laser pointers, burns from petrol bombs and bruises and cuts from flying.
Protesters hurled bricks at officers and ignored warnings to leave before tear gas was deployed in the Sham Shui Po area, police said, calling a march there an "unauthorised assembly".
Tear gas was also deployed in central Hong Kong on both sides of Victoria Harbour, in the Tsim Sha Tsui area on the Kowloon side and in Wan Chai on Hong Kong Island.
At one point, protesters blocked the entrance to a plaza to prevent police from entering.
A railway station in Kwai Fong filled with smoke after about a dozen police officers fired tear gas inside.
It was not clear how many protesters were inside the station at the time, but it has been rare for officers to fire tear gas indoors.
Earlier, a large group of mostly young protesters marched down the middle of Hennessey Road, a main shopping drag in the Causeway Bay area, as a rally was held in nearby Victoria Park.
Many wore face masks to shield their identities, and a few had helmets.
Others just carried backpacks over the black T-shirts that have become their uniform.
The protest movement's demands include the resignation of the Chinese territory's leader, Carrie Lam, democratic elections for her successor, the release of those arrested in earlier protests and an investigation into police use of force.
A former British colony, Hong Kong was returned to China in 1997 under the principle of "one country, two systems", which promises the city certain democratic rights not afforded to people on the mainland.
But in recent years, some have accused the Communist Party-ruled central government of steadily chipping away at their freedoms.
- What has been happening in Hong Kong?
Pro-democracy campaigners have been staging demonstrations across the city over the weekend, descending into violent clashes with police.
On Sunday, police fired tear gas into a railway station at Kwai Fong station.
A number of people were reportedly hurt in the clashes, including a woman who was said to have been blinded after allegedly being fired at by police.
Protesters at the airport wore eye patches as a marker of the violence.
The city is now in its tenth week of protests, which started after the government attempted to introduce an extradition bill which would mean criminal suspects could face trial in mainland China.
Since then, Hong Kong's chief executive Carrie Lam has said that the bill is "dead".
The demonstrations have since morphed to include Ms Lam's resignation, democratic elections, the release of those arrested in earlier protests and an investigation into police use of force against the protesters.
No end appears to be in sight for the protests, with neither side conceding.
Beijing, which ultimately controls the semi-autonomous region, has warned protesters it is "only a matter of time" before they are punished for their dissent.
Hong Kong's flagship carrier, Cathay Pacific, is feeling the brunt of the protests.
Its share value dropped after it sided with Beijing's orders to suspend staff taking part in the protests from working on flights to mainland China.
Some 75 million people used Hong Kong Airport last year, making it the eighth busiest in the world.