HONG KONG — Riot police armed with pepper spray and batons clashed with protesters late Tuesday at Hong Kong’s airport, bringing violence to the key international hub while passengers remained stranded inside after many departing flights were canceled.

The police intervened after a frenzied mob of protesters seized a man, claiming he was an undercover Chinese police officer, and blocked paramedics from transporting him for hours after he lost consciousness. Later Tuesday night, the group seized a second Chinese man, identified by his documents and his editor as a reporter for the Global Times, the Chinese Communist Party-run newspaper.

Weighing in on the developments, President Donald Trump tweeted: “Our Intelligence has informed us that the Chinese Government is moving troops to the Border with Hong Kong. Everyone should be calm and safe!” He did not elaborate.

The incidents marked a brazen escalation by protesters in confronting those they perceive as infiltrators from mainland China and capped a second consecutive day of chaos at Hong Kong’s airport, among the busiest in the world. Authorities have been unable to quell months of dissent in the financial hub.

After mass cancellations Monday evening, flights had been gradually returning to normal throughout Tuesday, even as thousands of black-clad demonstrators returned to occupy parts of the airport, carrying placards denouncing police brutality and calling for freedom for Hong Kong.

But by late afternoon, with protesters using luggage carts as makeshift barricades and blocking passengers from reaching the departure gates, causing long lines, authorities said they were suspending check-in at both of the airport’s terminals.

Arguments erupted between frustrated passengers and protesters, with some stranded passengers crying and saying they just wanted to get home.

Man beaten

A more disturbing scene began taking shape later in the day when a group of protesters surrounded a man they believed to be an undercover police officer from Shenzhen, the Chinese city across the border from Hong Kong. There was no confirmation of the man’s identity or profession, but the protesters cable-tied his hands and did not let him move or leave for hours.

Those closest to the melee held their phones aloft and tried to film the struggling man, while some jeered and laughed at him. Protesters held a handmade sign over the man that read in English, “I am China’s police. I pretend to be protester,” as he appeared to struggle to remain conscious.

Police entered the airport to help the man, whom paramedics tried to remove on a stretcher. The presence of officers sparked chaos, as protesters spilled out of the airport and began attacking police vans with officers inside.

At one point, an officer was overwhelmed and his baton taken by protesters, who beat him with it. The group retreated only after the officer appeared to pull his gun from its holster.

After the standoff with police, a second man was apprehended by protesters and bound to a luggage cart with cable ties. Protesters removed his passport, bank cards and other personal documents and scattered them on the floor. The documents identified him as a reporter at the Global Times.

Hu Xijin, the newspaper’s editor in chief confirmed on Twitter that the second man seized by protesters was a reporter for the Chinese state outlet.

“Fu Guohao, reporter of GT website, is being seized by demonstrators at HK airport,” Hu said. “I affirm this man being tied in this video is the reporter himself. He has no other task except for reporting. I sincerely ask the demonstrators to release him. I also ask for help of West reporters.”

Trump on Tuesday described the developments in Hong Kong as a “very tough situation.”

“I hope it works out for everybody, including China, by the way,” Trump told reporters. “I hope nobody gets hurt. I hope nobody gets killed.”

The State Department urged China to adhere to its commitment to allow “a high degree of autonomy” in Hong Kong — part of an agreement under which Britain relinquished its former colony.

“We condemn violence and urge all sides to exercise restraint, but remain staunch in our support for freedom of expression and freedom of peaceful assembly in Hong Kong,” the department said, adding that the protests reflect Hongkongers’ “broad concerns about the erosion of Hong Kong’s autonomy.”

The demonstration at the airport started as a peaceful one against police use of force in response to recent protests. Chants of “Return the eye!” broke out — a reference to an incident Sunday night when a young woman was shot in the eye, possibly by a bean bag round, during a clash between police and protesters. Senior officers said Tuesday they were unsure how the woman was injured.

Tempers flare

Time and again, protesters scolded their peers when tempers flared. Chants of “Lang jing!” — Calm down! — rang through the departure hall when arguments threatened to boil over.

Roving teams of protesters handed snacks to stranded passengers and appealed for understanding as they distributed pamphlets detailing their case against police brutality. Others bowed repeatedly and said “sorry” without yielding their ground in the face of angry passengers.

“Sorry for inconvenience. We have no choice,” said a sign held in front of a barricade blocking departures.

Police said they were closely monitoring the situation at the airport, working with airport authorities, and would carefully consider the need to use force.

Pavol Cacara, a Slovakian machinery importer who faced off with protesters, said his flight to Istanbul was canceled once already.

“You cannot make freedom by taking freedom from others!” he bellowed, shaking with rage, at a mass of young demonstrators in black T-shirts. “This is what the Chinese want you to do, to make you lose support of the world. You are helping them!”

Stranded by the turmoil

Some passengers were sympathetic. Krishna Hariharan, a 27-year-old IT engineer from Chennai, India, said his five-day holiday in Hong Kong was already extended to seven days because of canceled flights, angering his boss. He said he had to sleep in the terminal because he was running out of money.

But he praised a group of protesters who had come over to apologize and give him bottled water and biscuits.

“I can’t blame anyone,” Hariharan said. “They are seeking justice, and it just happens that our fates are intertwined like this. If the government comes down hard on them — then what are they governing for?”

But the apprehension of two Chinese men later that night raised the stakes amid an already delicate situation between Hong Kong and authorities in Beijing who have sovereignty over it.

Statements from Chinese government officials and state media have grown steadily more shrill before Tuesday, accusing protesters of “terrorism” and warning of an impending crackdown in the semiautonomous financial center.

The political crisis, triggered by now-suspended plans to allow extraditions to mainland China, has swollen as Hong Kongers demand the bill’s full withdrawal, an independent inquiry into police actions toward protesters, greater democracy and an amnesty for those arrested in clashes between demonstrators and police.