At airport Monday, officials had halted all departures, affecting tens of thousands of passengers.

Thousands of protesters shut down Hong Kong's international airport Monday, defying an intensifying police crackdown as China issued ominous warnings that described the protests as "terrorism" and began massing a paramilitary force in a southern border city.

Fears have been mounting that Beijing - squeezed by the trade dispute with the United States and approaching a nationwide celebration of the founding of the People's Republic of China - would soon resort to military action to quell the pro-democracy protests in the semiautonomous territory. Chinese officials and state news media actively stoked those fears on Monday. "The radical demonstrators in Hong Kong have repeatedly attacked police with extremely dangerous tools in recent days, which constitutes a serious violent crime, and now they are descending into terrorism," said Yang Guang, a spokesman for the Hong Kong and Macao Affairs Office in Beijing. It was the first time the office had portrayed the protests in Hong Kong as "terrorism."

"We should relentlessly crack down on such violent criminal acts without mercy, and we firmly support Hong Kong police and judicial authorities in bringing the criminals to justice as soon as possible," Yang told reporters from state and Hong Kong media.

The nationalist Global Times tabloid tweeted a video showing Chinese armored personnel carriers heading toward the southern city of Shenzhen, which borders Hong Kong, ahead of what the paper called "large-scale exercises" by the People's Armed Police, a paramilitary unit. "The tasks and missions of the Armed Police include participating in dealing with rebellions, riots, serious violent and illegal incidents, terrorist attacks and other social security incidents," the newspaper elaborated in an accompanying story.

And China's state broadcaster, CCTV, issued a commentary Monday headlined "Alert! There are signs of terrorism on the streets of Hong Kong." warned: "No country can accept terrorist acts in its own country. . . . Hong Kong has reached an important juncture. 'End violence and restore order' is the most important, urgent and overriding task of Hong Kong at present!"

Earlier, the Chinese government department responsible for Hong Kong held its third news conference in three weeks - having not held a briefing in the 22 years since Britain returned the territory to the mainland.

Some of the protesters who had been occupying the airport's arrivals hall swarmed into the departures area Monday, prompting authorities to cancel all flights and advise travelers to leave one of the world's busiest hubs. Airport operations resumed Tuesday morning, though there were some delays and cancellations stemming from the previous night's disruption.

Monday's shutdown came in response to a sharp increase in the level of force employed by Hong Kong's embattled police. Hours before the airport shutdown, two police officers elsewhere in the city had pinned a black-clad demonstrator to the concrete, one officer's knee pressing the young man's face into a pool of his own blood.

"I've already been arrested," the man yelled as he cried for help. "Don't do this, I'm begging you."

The scene, captured Sunday night by a cameraman from the Hong Kong Free Press, was jarring even in a city now accustomed to weekends awash with tear gas. It unleashed a fresh wave of anger toward Hong Kong's police force, and the government more broadly, spurring thousands of demonstrators to respond by occupying the airport.

At the airport Monday, officials had halted all departures by late afternoon, affecting tens of thousands of passengers.

Hong Kong's airport authority said flights were fully suspended Monday at about 3:30 p.m. local time (3:30 a.m. EDT), affecting tens of thousands of passengers. The authority said it was working on restarting operations from 6 a.m. local time on Tuesday morning.

After sitting in the arrivals hall for much of the day, many protesters began leaving the airport in the evening amid rumors on social media and messaging apps that police were preparing for a large clearance operation. The protesters, many dressed in black, streamed across the roads around the airport, bringing traffic to a near-standstill. Some travelers abandoned buses and taxis and wheeled bags through the traffic. Many said they were headed to a nearby bus station.

On Sunday night, Hong Kong police intensified their crackdown with new and more aggressive tactics after more than two months of sustained protests and more than 600 arrests.

Officers disguised themselves as protesters to arrest suspects, launched tear gas inside a subway station and fired on protesters at close range with less-than-lethal ammunition. One young woman was shot in the face with what appeared to be a bean bag round, severely injuring her eye. Police said Monday that the videos and photos had to be verified and that they could not confirm "the reasoning behind this lady's injury."

But the incident provided the latest rallying point for protesters.

"The police have had enough, to be honest," said Clement Lai, a former police superintendent who now runs his own security firm. "They feel like they have been bullied for two months now, and they knew themselves more than capable to use real force and tactics to control the situation."

"If the order was given that they need to escalate their action and their force, these guys are more than happy to do that."

Mel, 40, who took part in the airport demonstrations and carried a sign showing pictures of bloodied protesters, said she wanted "to show the world that what we are looking for is freedom."

She said she was angry about the "dirty methods" police used on Sunday night and early Monday morning.

Mel, who gave only her first name, added that a decision was made among many protesters to leave early Monday evening because of fears of police forcibly clearing the airport.

The police actions appear to be part of broader efforts by the Hong Kong government, with support of officials in Beijing, to end the political crisis, through an approach that includes ramping up pressure on businesses, leveling heavy charges against arrested protesters and using state-controlled media to pump out increasingly shrill, conspiratorial claims about who is organizing the demonstrations.

"After a period of several weeks of uncertainty as to who was coordinating the government response, last week saw the rollout of Beijing's multipronged, comprehensive strategy to deal with the protests," said Sebastian Veg, a historian of China and professor at the School of Advanced Studies in Social Sciences in Paris. "It consists in uniting all forces with whom common ground can be found to isolate and defeat the enemy."

He added: "The aim is to turn public opinion against the protests by drastically raising the cost of participation."

The new police tactics came after former deputy police commissioner Alan Lau was called out of retirement last week to help the embattled force.

Lai credited the new approach, in part, to the return of Lau to the force he left in 2014. "He is coming back with a mission," the former superintendent said.

Hospital officials said that 45 people were injured in weekend protests and that 25 remain hospitalized. Two were in serious condition.

One police officer who has worked on the front lines over the past month said officers' new ploy of disguising themselves as protesters - wearing masks and yellow hard hats, in black civilian clothes - was a deliberate tactic from the police Special Duties Unit, nicknamed "The Flying Tigers," to sow mistrust among protesters.

This is a tactic they will continue to use, the officer said, speaking on the condition of anonymity because he was not authorized to speak to reporters. Police on Monday also displayed trucks mounted with water cannons that they could deploy to disperse crowds.

One 22-year-old protester who has been on the front line for weeks said the more aggressive moves by police had caught some demonstrators off guard and yielded results.

"It was quite effective for them; they are changing their strategy," he said. "We know now the police have no limits. They will not follow the rules and the law."

The government, in what has become a weekly ritual, condemned protesters on Monday and said a police officer was injured after being hit with a firebomb tossed by a demonstrator.

Protests began earlier this year over the government's attempts to push through a bill that would allow extraditions to mainland China. The legislation, which numerous critics said would be a severe blow to Hong Kong's autonomy, was suspended by Hong Kong leader Carrie Lam in June.

Lam, however, has refused to fully withdraw the measure. She has issued apologies as well as condemnation, none of which have quelled the crisis. Most recently, she has pivoted to focus on how the unrest is damaging Hong Kong's economy.

Protesters have offered a list of five demands that has shifted slightly in recent weeks. Much of the focus is now on the creation of an independent commission to investigate the handling of the bill and the subsequent fallout.

An inquiry has drawn wide support, with the Hong Kong General Chamber of Commerce, law groups and civil society organizations backing its creation, but the government continues to resist such calls. Lam has said she thinks an in-house investigation by police of their actions is sufficient and has not addressed the other demands.

The front-line protester said the police's new strategy would only harden those who have already dedicated themselves to the fight.

"You can see our equipment - shields, helmets - is for defense, not for offense," he said. "From now on, I think that will change. Some types of weapons will be used. We are standing there and getting beat by them."

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The Washington Post's Anna Fifield, Shibani Mahtani and Tiffany Liang contributed to this report.