Photo: CNN

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 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 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 were able to 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.

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 tried 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."

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.