HONG KONG—Police clashed with protesters thronging Hong Kong’s airport late Tuesday, with violence flaring at the end of a second straight day of pro-democracy demonstrators overwhelming one of the world’s busiest travel hubs.

Without citing specific evidence, President 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!”

U.S. defense officials declined to confirm Mr. Trump’s assertion of a buildup of forces along Hong Kong’s border. One military official said Chinese authorities appeared to be wary of using military force to quell the unrest at the moment, cognizant of the possibly severe consequences from such a move.

Social-media accounts run by Chinese state news outlets published on Monday footage of mainland paramilitary police forces arriving in the southern city of Shenzhen, which abuts Hong Kong.

At the airport Tuesday, demonstrators seized two men they suspected of being pro-Beijing infiltrators, and witnesses said they were beaten. In both cases, paramedics had to struggle to extract them from the crowd.


“Everything is out of control right now,” said one 21-year-old protester. “This movement doesn’t have a leader; everyone does what they want.”

Outbound and inbound flights resumed Wednesday morning. Hong Kong’s airport authority said Wednesday it had secured a court injunction “to restrain persons from unlawfully and wilfully obstructing” the airport’s activities. Local police said Wednesday they had arrested five people for “offences including unlawful assembly, possession of offensive weapons, assaulting police officers and breaching of the peace.” They said two police officers were injured.

Tuesday’s escalation at the airport amounts to one of the biggest challenges to authorities in Hong Kong and Beijing since the beginning of the city’s summer of discontent. For President Xi Jinping of China, the conflict presents a thorny political dilemma as he grapples with a slowing economy and a trade fight with Washington just months before lavish celebrations of the 70th anniversary of Communist Party rule on Oct. 1.

A crackdown would spark international criticism and put Hong Kong’s fragile status at further risk. In a morning news conference, the city’s leader, Carrie Lam, choked back tears and warned that protesters were dragging Hong Kong into “the abyss.”


The day’s spiraling violence showed how volatile the protests are becoming. Hong Kong authorities have begun to respond more severely, with business leaders and officials offering stronger support to quell the unrest as it starts to damage Hong Kong’s economy and its global reputation as a safe place to do business.

On mainland China’s Twitter -like microblogging service Weibo, users showed footage of one of the men who had been beaten in which he appeared to be unconscious and breathing through an oxygen tube. Many users expressed shock and anger at the assault in online comments.

In comments earlier on Tuesday, Mr. Trump called the developments in Hong Kong a “very tough situation” and said he hoped nobody would be hurt or killed. “I hope it works out for everybody, including China, by the way,” the Republican president said.

A senior administration official said the U.S. believed Chinese military intervention in Hong Kong would depend on whether Beijing determined that local authorities had lost control of the crisis.


Meanwhile, Secretary of State Mike Pompeo and China’s most senior foreign-policy official, Yang Jiechi, met in New York Tuesday morning. While the State Department didn’t reveal any details of the conversation, spokeswoman Morgan Ortagus said the two officials “had an extended exchange of views on U.S.-China relations.”

At the airport, the day began with small numbers of black-clad and often masked protesters dotting the arrival hall, some playing the violin and singing. It ended with fierce fights, including one in which protesters grabbed an officer’s baton and began beating him with it until he pulled his gun and they ran off.

The airport suspended all check-in activity and advised people not to travel there as the protesters’ numbers swelled quickly late in the afternoon. Travelers who tried to pick their way through the crowds were shouted down and cursed at by protesters. A few passengers managed to push their way through.

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The mood turned uglier in the evening as protesters turned on a young man among them whom they suspected of being an undercover police officer. Protesters searched his pockets and said they found an identification card bearing a name matching one in an online database of police officers in Shenzhen.


At one point, some protesters suggested they would let the man go but others dissented. A handful of airport officials attempted to intervene but were rebuffed. A small group of protesters tied the man up and shuffled him around the stiflingly hot airport hall for three hours, chanting “Bear the consequences.”

Later in the evening a second man was seized by protesters and accused of being in league with the police. Although he was wearing a press jacket, protesters said he acted suspiciously. They detained him and emptied his bags, finding a blue shirt emblazoned with a slogan supporting the police and a business card of a member of a pro-Beijing organization.

Protesters surrounded him and beat him as he lay on the floor. The editor in chief of the Global Times, a pro-government Chinese tabloid, tweeted that the man was a reporter with the paper and asked the protesters to release him. A team of medics made their way to the injured man, who was hauled onto a stretcher and carried out him to a waiting ambulance.

Demonstrators flooded the airport's arrival and departure halls for a second straight day, snarling the busy transit hub and frustrating travelers. Photo: Thomas Peter/Reuters

Hong Kong’s airport presents an easy target for demonstrators. Its open design admits large numbers of people quickly and efficiently, and the city prides itself on the ease of access. In addition to cabs, there are frequent buses as well as express trains that run every 10 minutes and take less than half an hour to make the trip from the city center.

Airport authorities implemented new restrictions early Tuesday, requiring travelers to show proof of their bookings before letting them into the check-in area. The arrival of protesters in greater numbers by the afternoon overwhelmed those efforts.

“The police have correspondent measures to support the airport authority in monitoring the situation, and we’ll pay attention to the acts of these protesters and whether there is any breach of the peace inside the airport,” a police spokesman said at a news conference Tuesday before the situation deteriorated.

British Airways and Virgin Atlantic offered travelers scheduled on flights departing Hong Kong this week the option to rebook for later in the month. British Airways said it would offer customers the option of a full refund.

A spokesman for Lufthansa, the German flag carrier, said that its flights departing Hong Kong Tuesday for Munich and Frankfurt would continue as scheduled. But due to the airport disruption at the departures security gate, only passengers connecting in Hong Kong from other cities would be able to board.

Tuesday’s disruptions were a further blow to Cathay Pacific, Hong Kong’s flag carrier, which has come under heavy pressure from China over employees who join the protesters.

On Monday, the airline said it would fire staff who participated in unauthorized protests. On Tuesday, parent company Swire Pacific said it shared the Chinese government’s view of the city’s future and condemned what it called illegal acts that threatened the “one country, two systems” principle under which Hong Kong maintains some autonomy under Chinese rule.

“We resolutely support the Hong Kong SAR Government, the Chief Executive and the Police in their efforts to restore law and order,” Swire said in a release.

Cathay’s shares fell 2.6% Tuesday.

An overwhelming turnout a day earlier forced the airport to halt all outbound flights. The airport was still contending with those cancellations Tuesday, with hundreds of morning and afternoon flights taken off the board and dozens of others delayed.

Hong Kong Airport Faces Second Day of Disruption The airport suspended all passenger check-ins and advised people not to go to the airport An injured man who was suspected by protestors of being a Chinese spy is taken away by paramedics at Hong Kong’s international airport late Tuesday. Hundreds of flights were cancelled or suspended as pro-democracy protesters staged a second disruptive sit-in at the sprawling complex. MANAN VATSYAYANA/Agence France-Presse/Getty Images Anti-government protesters surround a man they accuse of being a mainland Chinese police officer during a sit-in against police violence in Hong Kong’s international airport on Tuesday. LAUREL CHOR/EPA/Shutterstock A police officer takes a fall during a scuffle with pro-democracy protesters during a second day of demonstrations at Hong Kong’s airport on Tuesday. Manan VATSYAYANA/Agence France-Presse/Getty Images Riot police trying to disperse protesters taking part in a second day of mass demonstrations at Hong Kong’s airport on Tuesday. thomas peter/Reuters Protesters at Hong Kong’s airport on Tuesday detained a man they suspected of being an undercover police officer. Paramedics had to struggle to extract him from the throng. Vincent Yu/Associated Press Riot police using pepper spray to disperse protesters at Hong Kong’s airport on Tuesday. tyrone siu/Reuters Riot police aiming pepper spray at protesters who brought Hong Kong’s airport to a standstill for a second day on Tuesday. tyrone siu/Reuters Antigovernment protesters try to prevent a passenger from breaching a barricade in front of departure gates. Thomas Peter/Reuters

A traveler hands her luggage to security guards as she tries to enter the departure gates during an second day of demonstrations by prodemocracy protesters at Hong Kong's airport. Philip Fong/Agence France-Presse/Getty Images A member of airport staff trying to contain antigovernment protesters during Tuesday’s demonstration. Airport authorities advised people not to go to the airport. Issei Kato/Reuters Passengers waiting for information regarding their flights. The airport was still contending with Monday’s cancellations, with hundreds of morning and afternoon flights Tuesday taken off the board and dozens of others delayed. issei kato/Reuters A protester wears a mock eye patch. Demonstrators have been wearing eye patches in a reference to a woman who sustained an eye injury allegedly by police. laurel chor/epa/Shutterstock Demonstrators hold a sign apologizing to travelers and explaining why they are protesting. They have widened their list of demands and are calling for an inquiry into police conduct and a more democratic government. Kyle Lam/Bloomberg News

Protesters waved posters depicting alleged police brutality and chanted pro-democracy slogans.

“Protesting in the airport is the best way to tell the world what’s happening in Hong Kong,” said Jade Yai, 16, who had been handing out fliers alleging police brutality in the airport to travelers since the early morning.

Other core municipal facilities were also affected by protests Tuesday. Staff for at least eight public hospitals went out on strike. Hundreds of doctors, nurses and paramedics staged a walkout at Queen Mary, one of the city’s largest public hospitals, in protest of what they saw as police brutality during the previous weekend’s demonstrations.

Protests have embroiled Hong Kong this summer, sparked by a proposed law that would have allowed the Chinese government to extradite and prosecute Hong Kong citizens. A broad swath of the city’s residents rallied in opposition to the law.

Hong Kong’s government declared the bill dead in response, but thousands of Hong Kongers continue to demonstrate, calling for an inquiry into police conduct and a more democratic government.

By early Wednesday morning the number of protesters had dwindled to a few stragglers and the airport started to bounce back to life, with passengers arriving on red-eye flights walking out into the detritus of the day’s protest. Posters and handmade signs remained scattered on the floor.

A group of protesters huddled together on the floor of the departures level going over the personal items of one of the two men held by the mass of protesters hours earlier—credit cards, train receipts, scraps of paper, a Chinese passport and a blue T-shirt with a pro-police slogan.

One demonstrator, wearing a purple shirt and tie and a surgical mask, said he believed the protest was a success.

“We want to pressure the government to respond to our demands,” he said.

Asked if he believed the chaotic protest scenes and the taking of the two hostages could diminish support for their cause among moderate Hong Kongers, one demonstrator he said: “Yes we risk losing support, but if we handle this well we can win back the support and more.”

A fellow protester said the risk of losing support was worth the upside of disrupting the airport. “We have to do everything we can,” he said. “This is our last resort.”

—Preetika Rana, Stella Yifan Xie, Sara Germano, Chun Han Wong, Andrew Restuccia and John Lyons contributed to this article.

Write to Jon Emont at jonathan.emont@wsj.com and Mike Bird at Mike.Bird@wsj.com