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.

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 canceled. The authority advised people not to come to the airport, one of the world's busiest transport hubs.

More than 200 flights were canceled 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.

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.

With files from the Associated Press

Guest:

Zhaoyin Feng, Washington reporter for BBC Chinese; she tweets