Hong Kong's airport cancelled all flights on Monday, blaming demonstrators for the disruptions, while China said the anti-government protests that have roiled the city through two summer months had begun to show "sprouts of terrorism".
The airport authority said it was working with airlines to resume flights from 6 a.m. on Tuesday, but the developments raise the stakes after a weekend of skirmishes during which both activists and police toughened their stances.
Some Hong Kong legal experts say official descriptions of some protesters' actions as terrorism could lead to the use of extensive anti-terror laws and powers against them.
China's People's Armed Police also assembled in the neighbouring city of Shenzhen for exercises, the state-backed Global Times newspaper said.
People in the Asian financial hub responded by taking to the streets again, picketing a police station and returning in their hundreds to a metro station where police had hit activists with batons, to protest against the heavy-handed tactics.
The increasingly violent demonstrations have plunged the Chinese-ruled territory into its most serious crisis in decades, presenting Chinese leader Xi Jinping with one of his biggest popular challenges since he came to power in 2012.
"Hong Kong has come to a critical juncture," said Hong Kong and Macau Affairs office spokesman Yang Guang in Beijing.
"Protesters have been frequently using extremely dangerous tools to attack the police in recent days, constituting serious crimes with sprouts of terrorism emerging."
The protests began in opposition to a bill allowing extradition to the mainland but have widened to highlight other grievances, winning broad support.
The precise trigger for the airport's closure was not clear, since protesters occupying the arrivals hall for four days have been peaceful.
"This is about our freedom," said one of the thousands of protesters who remained there, a 24-year-old wearing a mask, who gave his name only as Yu. "Why should we leave?"
Hong Kong is the world's busiest air cargo port and the 8th busiest by passenger traffic, says the Airports Council International (ACI), a global association. It has been filled with anti-government protesters for four days.
The mostly young black-clad protesters have chanted slogans such as "No rioters, only tyranny!" and "Liberate Hong Kong!" while approaching travellers with flyers describing their demands and explaining the unrest.
Demonstrators say they are fighting the erosion of the "one country, two systems" arrangement enshrining some autonomy for Hong Kong when China took it back from Britain in 1997.
The activists at the airport have been polite and passengers mostly unperturbed. "I was expecting something, given all the news," one arrival, Gurinda Singh, told Reuters.
"I'm just pleased my plane arrived and the protests here seem peaceful."
Some activists moved to the departure area and caused disruptions, police told a news conference as the cancellations were announced.
Police declined to say if they would move to clear the demonstrators. There was no visible police presence in either the departure or arrivals area.
"Airport operations at Hong Kong International Airport have been seriously disrupted as a result of the public assembly at the airport today," the city's airport authority said in a statement, without elaborating.
About 190 flights were hit, Chinese aviation data firm VariFlight said, though planes already en route to Hong Kong were allowed to land.
Demonstrators threw up barricades across the city at the weekend, as police fired tear gas into crowded underground train stations as well as rubber bullets and pepper pellets at close range.
In response, protesters have sought to channel a Bruce Lee maxim: "Be water," employing a flash-mob strategy to frustrate authorities and stretch their resources.
Still, scores of protesters were arrested, sometimes after being beaten with batons and bloodied by police. One young female medic was hospitalised after being hit by a pellet round in the right eye.
Hundreds of people returned on Monday to the scene of some of the clashes to protest against the use of force.
"We are completely disappointed with Hong Kong police," said Terry Ng, 30, a social worker protesting outside Wan Chai police station. "The weight of their shots caused a young girl to lose one eye. Do they still have a conscience?"
Authorities have called the citywide demonstrations illegal and dangerous, while highlighting their impact on the already-faltering economy and residents' activities.
Beijing says criminals and agitators are stirring violence, but its use of the word "terrorism" on Monday was its starkest warning yet, possibly paving the way for a national-security response to the crisis.
China has used the threat of terrorism to justify tough measures in its regions of Xinjiang and Tibet, which have been criticized by rights groups and Western governments. It warned them off on Monday as well.
"Hong Kong is China's Hong Kong and Hong Kong affairs are purely China's internal affairs," foreign ministry spokeswoman Hua Chunying said in a statement.
China has also put pressure on big companies, such as Cathay Pacific Airways, whose shares tumbled to a 10-year low on Monday, after it was told to suspend staff engaged in illegal protests.
Police, who have arrested more than 600 people since the unrest began more than two months ago, demonstrated a water cannon.
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