Mass flight cancellations at Hong Kong's airport disrupted one of the world's busiest air transport hubs on Monday, as several thousand pro-democracy protesters peacefully demonstrated at the terminal for a fourth day.

Key points: The protests led to the cancellation of flights on Monday evening and into Tuesday morning

The airport said it expected to be able to restart flights from 6:00am Tuesday (local time)

In Beijing, the Cabinet's Hong Kong and Macao Affairs Office issued a statement saying the situation was "beginning to show the sprouts of terrorism"

The sit-in caused chaos for tourists and business travellers and played havoc with global airlines' flight schedules, in a fresh escalation of the city's protest movement that threatens the Asian city's business-friendly image.

The international airport said the demonstration had "seriously disrupted" airport operations and caused the cancellation of more than 150 flights.

Qantas cancelled three flights due to leave Hong Kong on Monday night and grounded a further three departing Australian cities on Tuesday morning — QF97 from Brisbane, QF29 from Melbourne and QF127 from Sydney.

Two Virgin Australia flights — VA82 to Sydney and VA68 to Melbourne — were due to depart the airport on Monday night and were re-scheduled for Tuesday.

Protesters hold up placards as they take part the sit-down rally. ( AP: Vincent Thian )

Traffic on roads to the airport was very congested and car park spaces were full, the airport said.

The airport said it expected to be able to restart flights from 6:00 am on Tuesday (local time).

Hong Kong has experienced more than two months of mass protests calling for democratic reforms and an independent inquiry into police conduct.

The increasingly violent protests have plunged the Chinese-ruled city into its most serious crisis in decades and presented a serious challenge to Beijing.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 51 seconds 51 s Protesters greet arrivals to Hong Kong

Authorities have arrested more than 600 people since the rallies began in June.

In Beijing, the Cabinet's Hong Kong and Macao Affairs Office issued a statement saying the situation in Hong Kong was "beginning to show the sprouts of terrorism" and constituted an "existential threat" to the city's population.

"One must take resolute action toward this violent criminality, showing no leniency or mercy," said the statement, attributed to spokesman Yang Guang.

Protesters surround banners that read: "Those who charge the street today are brave!" and "Release all the detainees!". ( AP: Vincent Thian )

"Hong Kong has reached an inflection point where all those who are concerned about Hong Kong's future must say 'no,' to lawbreakers and 'no' to those engaged in violence."

The British government called for calm from all sides.

A spokesman for Prime Minister Boris Johnson said Britain wanted the Hong Kong government to engage with all parties in constructive dialogue.

Some of the protesters' signs at the airport referred to a woman who was injured in her eye on Sunday during another night of violent clashes.

Local media quoted passers-by as saying that she was hit by a beanbag round fired by police in Tsim Sha Tsui and was sent to hospital for treatment.

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Hong Kong police have defended their tactics and use of force against protesters, including using tear gas inside a train station, deploying officers disguised as protesters and allegedly shooting at demonstrators at close range with pepper-spray pellets.

Deputy Commissioner Tang Ping-keung acknowledged that police used decoy officers in some operations but would not go into details.

Only flights that had already begun boarding or those cleared for landing were allowed to use runways at the airport. ( ABC News: Robert Koenig-luck )

At the airport, a flight attendant protesting on his day off, who gave only his surname, Lau, to avoid repercussions from his employer, told AP that heavy-handed police tactics had alienated some people.

"The police have told a lot of lies to Hong Kong people. We cannot believe them anymore. We have to come here to protest," he said.

China has reportedly threatened to bar air crew who take part in protests from its air space.

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