Beijing on Monday said protests in the Asian financial hub, which started as opposition to a now suspended extradition bill, had reached a "critical juncture".

"Protesters have been frequently using extremely dangerous tools to attack the police in recent days, constituting serious crimes with sprouts of terrorism emerging," said Hong Kong and Macau Affairs office spokesman Yang Guang in Beijing.

Hong Kong is the 8th busiest by passenger traffic, handling 73 million passengers a year. The airport has been filled with anti-government protesters for five days.

A Reuters reporter saw more than 100 travellers queuing up at Cathay's ticketing counter early on Tuesday.