HONG KONG - Riot police clashed with pro-democracy protesters at Hong Kong’s airport late Tuesday night, moving into the terminal where the demonstrators had shut down operations at the busy transport hub for two straight days.

Officers armed with pepper spray and swinging batons confronted the protesters who used luggage carts to barricade entrances to the airport terminal.

Police took several people into a police van waiting at the entrance to the airport’s arrivals hall.

Police said they tried to help ambulance officers reach an injured man whom protesters had detained on suspicion of being an undercover agent.

Protesters also detained a second man who they suspected of being an undercover agent. After emptying out his belongings, they found a blue T-shirt that has been worn by pro-Beijing supporters that they said was evidence he was a spy.

Earlier in the day, authorities were forced to cancel all remaining flights as the city’s pro-Beijing leader warned that the protesters had pushed events onto a “path of no return.”

After a brief period early in the day when flights were able to take off and land, the airport authority suspended check-in services for departing flights as of 4:30 p.m. Departing flights that had completed the process would continue to operate.

It said it did not expect arriving flights to be affected, although dozens were already cancelled. The authority advised people not to come to the airport, one of the world’s busiest transport hubs.

More than 200 flights were cancelled Monday and the airport was effectively shut down with no flights taking off or landing. Passengers have been forced to stay in the city while airlines struggle to find other ways to get them to their destinations.

For Grace Bendal, a 43-year-old contractor from the Philippines, Tuesday was the second straight day she came to the airport only to learn flights were cancelled. She spent the weekend in Hong Kong with her primary school-age children, who were eager to return to classes.

She said they have already missed two days of school and the extra day in the city has cost her around 3,000 Hong Kong dollars ($400). Though there were no airline employees at check-in counters Tuesday evening, Bendal said she and her children planned to stay at the airport all night.

“I cannot blame them, because they are fighting for something,” Bendal said of the protesters. “But then it’s not right if we are the ones suffering. So I hope they give us a chance to go home.”

The airport disruptions are an escalation of a summer of demonstrations aimed at what many Hong Kong residents see as an increasing erosion of the freedoms they were promised in 1997 when Communist Party-ruled mainland China took over what had been a British colony.

The protests have built on an opposition movement that shut down much of the city for seven weeks in 2014 before it eventually fizzled and its leaders were jailed on public disturbance charges.

The central government in Beijing has ominously characterized the current protest movement as something approaching “terrorism” that poses an “existential threat” to citizens.

While Beijing tends to define terrorism broadly, extending it especially to nonviolent movements opposing government policies in minority regions such as Tibet and Xinjiang, its use of the term in relation to Hong Kong raised the prospect of greater violence and the possible suspension of legal rights for those detained.

Hong Kong leader Carrie Lam said the instability, chaos and violence have placed the city on a “path of no return.”

The black-clad demonstrators have shown no sign of letting up on their campaign to force Lam’s administration to respond to their demands, including that she step down and scrap proposed legislation under which some suspects could be sent to mainland China, where critics say they could face torture and unfair or politically charged trials.

Lam has rejected all calls for dialogue, part of what analysts say is a strategy to wear down the opposition movement through police action while prompting demonstrators to take more violent and extreme actions that will turn the Hong Kong public against them. At the airport, protesters discussed among themselves whether they should simply block all access to the facility.

Meanwhile, paramilitary police were assembling across the border in the city of Shenzhen for exercises that some saw as a threat to increase force against the mostly young protesters who have turned out by the thousands in the past 10 weeks.

While China has yet to threaten sending in the army — as it did against pro-democracy protesters in Beijing in 1989 — the Shenzhen exercises were a sign of its ability to crush the demonstrations, even at the cost to Hong Kong’s reputation as a safe haven for business and international exchange. Images on the internet showed armoured personnel carriers belonging to the People’s Armed Police driving in a convoy Monday toward the site of the exercises.

The People’s Liberation Army also stations a garrison in Hong Kong, which recently released a video showing its units combating actors dressed as protesters. Hong Kong police also put on a display of water cannons.

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Police have arrested more than 700 protesters since June and say they have infiltrated the demonstrators, leading to concerns that officers were inciting violence. Scores of protesters and police have been hurt, including a woman reported to have had an eye ruptured by a beanbag round fired by police during clashes Sunday.

Police said they are investigating the incident, which protesters have taken up as a rallying cry. Some of those joining in the airport occupation wore gauze bandages dyed with artificial blood over one eye.

The U.N.’s top human rights official condemned violence around the protests and urged both sides to settle their dispute through “open and inclusive dialogue.”

Rupert Colville, spokesman for U.N. High Commissioner for Human Rights Michelle Bachelet, said her office had reviewed evidence that police are using “less-lethal weapons in ways that are prohibited by international norms and standards.” That includes firing tear gas canisters into crowded, enclosed areas and directly at individuals, “creating a considerable risk of death or serious injury,” Colville said in a statement.

In a sign of rising tensions, protesters in the evening detained a man they claimed was a police officer from mainland China. They tied his wrists using plastic strips and poured water over his head. Airport security guards were present but did not appear to be able to stop the crowd.

Sally Tong, an 18-year-old protester, said they needed to keep holding him as evidence that mainland Chinese authorities are in Hong Kong to monitor the demonstrations. Tong said the man was dressed in black and wore a mask to look like one of them.

“We want to keep him here and investigate,” Tong said. Protesters said the man dropped his wallet when he was running away from them, and they found ID cards from mainland China and also found his name on a list of police officers online.

The Associated Press could not independently verify his identity.

After two months, the protests have become increasingly divisive and prompted clashes across the city.

Demonstrators have recently focused on their demand for an independent inquiry into what they call the police’s abuse of power and negligence. That followed reports and circulating video of violent arrests and injuries sustained by protesters.

Some protesters have thrown bricks, eggs and flaming objects at police stations. Police say several officers have suffered burns, bruises and eye damage from protesters.

Lam said dialogue would only begin when the violence ended. She reiterated her support for police and said they have had to make on-the-spot decisions under difficult circumstances, using “the lowest level of force.”

“After the violence has been stopped, and the chaotic situation that we are seeing could subside,” Lam said. “I as the chief executive will be responsible to rebuild Hong Kong’s economy ... to help Hong Kong to move on.”

She did not elaborate on any conciliatory steps her government will take.

The early protests were in specific neighbourhoods near government offices. However, the airport protest has had a direct impact on business travel and tourism. Analysts said it could make foreign investors think twice about setting up shop in Hong Kong, which has long prided itself as being Asia’s leading business city with convenient air links across the region.

“I don’t think I will ever fly to Hong Kong again,” said Kerry Dickinson of South Africa.

The protesters held up signs in Chinese and English to appeal to travellers from mainland China and elsewhere. “Democracy is a good thing,” said one sign in simplified Chinese characters, which are used in mainland China instead of the traditional Chinese script of Hong Kong.

Adding to the protesters’ anger, Hong Kong’s Cathay Pacific Airways told employees the carrier has a “zero tolerance” for them joining “illegal protests” and warned they could be fired.

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Associated Press photographer Vincent Thian in Hong Kong contributed.