Tuesday saw some of the most combative scenes since Hong Kong's protest movement began several months ago, with protesters detaining people at the airport, including a mainland Chinese journalist.
The city's chief executive warned that Hong Kong was "on the brink of no return." Five people were arrested and two police officers were injured, Hong Kong police said in a statement early Wednesday.
The police made their first significant appearance at the airport since protests began there five days ago, with at least five police buses arriving at the airport after 10:30 p.m.
Protesters scrambled to set up barricades as officers left the buses and entered the airport. Riot police appeared soon after, amassing outside the terminal, clashing with demonstrators.
In one confrontation, a riot police officer was attacked from behind as he held down a protester. His baton was then taken and used against him by a protestor, before the officer drew what appeared to be a pistol and aimed it at the crowd.
The police arrived at the airport during a standoff between protesters and paramedics, who were trying to reach a man accused by demonstrators of being an undercover police officer. The man appeared to have lost consciousness at one point, but protesters refused to allow paramedics to evacuate him for several hours.
Protester lines broke when police arrived, allowing paramedics to successfully evacuate him.
Police said in an earlier statement that their arrival to the airport was "not an operation to disperse those assembled but is for extricating the visitor safely."
"A visitor was assaulted and is currently being besieged by a large group of protestors at the Hong Kong International Airport. He requires immediate medical attention but the protestors concerned have been obstructing ambulance officers from rendering medical assistance," the statement said.
Another man was detained and zip-tied to a luggage cart by protestors. He was identified as mainland Chinese reporter Fu Guohao for state-run tabloid newspaper Global Times, according to the outlet's editor-in-chief.
He was later seen being wheeled out of the airport by first-aid workers.
Police eventually retreated from the airport, after making arrests outside the airport and deploying pepper spray multiple times. But hundreds of protesters remained late Tuesday.
The outbreaks of violence came after Hong Kong's Airport Authority announced that all check-in services were suspended for another night because terminal operations were "seriously disrupted."
"Members of the public are advised not to come to the airport," the authority said in a statement. All outbound flights which have not yet completed the check-in process have been canceled.
A government spokesperson described what happened at the airport as overstepping "the bottom line of a civilized society." Police would take "relentless enforcement action" to bring the culprits of violence to justice, the spokesperson said.
US President Donald Trump weighed in on the deteriorating situation on Twitter on Tuesday afternoon. He claimed that US intelligence showed "the Chinese Government is moving troops to the Border with Hong Kong. Everyone should be calm and safe!" China already keeps some troops garrisoned near the border.
A senior US administration official said the US was monitoring the unrest in Hong Kong and encouraged "all sides to remain calm, safe, and peaceful."
"As the President has said: 'they're looking for democracy and I think most people want democracy,' " the official said. "Freedoms of expression and assembly are core values that we share with the people of Hong Kong and these freedoms should be protected."
UK Foreign Minister Dominic Raab said he was concerned by "what's happening in Hong Kong and the worrying pictures of clashes between police and protesters at the airport."
"As I said to Carrie Lam during my call last week, we condemn the violence and encourage constructive dialogue to find a peaceful way forward," Raab tweeted.
Weeks of protests
Hong Kong's protest movement kicked off in earnest in June, sparked by a bill that would allow extradition to China.
Since then, the protests have expanded, with protesters now demanding greater democracy and an inquiry into alleged police brutality.
The demonstrations -- which on Sunday occurred for a 10th straight weekend -- have seen protesters and police clash numerous times, with police firing multiple rounds of tear gas.
On Tuesday morning, Hong Kong's Chief Executive Carrie Lam warned that the city was on "the brink of no return" and said it would take a long time to recover from the recent mass demonstrations.
Beijing, meanwhile, has criticized the actions of the protesters with increasing heat. On Monday, a top Chinese official said the protests "had begun to show signs of terrorism."
Last year, Hong Kong's airport handled 74.7 million passengers -- an average of about 205,000 per day. But on Monday, the airport was brought to a standstill as demonstrators occupied parts of the airport, in protest against police violence. Dozens of outgoing flights were canceled.
Many passengers were left stranded with little information as to their situation, and some even spending the night in the airport. Some expressed frustration and confusion to CNN.
Waiting at Hong Kong subway station, Loic, a 33-year-old French man who lives in Hong Kong, said that he didn't know if his flight was canceled. "I don't know what I can do. Maybe I can go to Shenzhen," he said.
But despite the days of disruption, others were supportive of the protest movement. "It touched me to see Hong Kong like this, I'm not angry," a 31-year-old passenger, who asked not to be identified, told CNN. "I still support them."
CNN's Sarah Dean reported from London. Kevin Liptak and Darran Simon contributed to this report
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