Stranded passengers look at a flight information board at the Hong Kong airport

Flights resumed Tuesday at Hong Kong airport a day after a massive pro-democracy rally there forced the shutdown of the busy international transport hub.

Early Tuesday, passengers with luggage were being checked in at the departures hall and information boards showed several flights were already boarding or about to depart.

The abrupt shutdown of one of the world's busiest hubs came after thousands of black-clad demonstrators flooded the airport for a peaceful rally.

It was a dramatic escalation of a 10-week crisis that has been the biggest challenge to Chinese rule of Hong Kong since the 1997 British handover.

The protests have infuriated Beijing, which described some of the violent demonstrations as "terrorism".

Washington overnight urged all sides to refrain from violence, as the crisis sparked by a bill to allow extradition to mainland China continues with no apparent end in sight.

'An eye for an eye'

On Tuesday morning, only a handful of protesters remained in the airport. But fears of a police operation to clear the facility overnight proved unfounded, with demonstrators simply leaving by themselves.

Many of the posters and artwork they had hung throughout the facility during the hours-long rally had been taken down, but graffiti -- some reading "an eye for an eye" -- could still be seen in several places.

The protesters adopted the slogan after a woman suffered a serious facial injury that reportedly caused her to lose the vision in one eye at a demonstration that turned violent on Sunday night.

The demonstrators accuse police of causing the injury by firing a bean-bag round, and cite the case as evidence of what they say has been an excessive and disproportionate response by police to their protests.

The activists have called on their supporters to return to the airport later on Tuesday, though it was unclear whether authorities would allow that to happen.

The protests that began in opposition to a bill allowing extradition to the mainland has morphed into a broader bid to reverse a slide of freedoms in the southern Chinese city.

But the city's Beijing-backed leader has ruled out meeting the protesters' demands, which include the right to choose their next leader and an investigation into alleged police brutality.

The demonstrations have become increasingly violent, with police using tear gas and rubber bullets to push back protesters who have sometimes hurled bricks and bottles.

"It is becoming more and more dangerous, but if we don't still come out at this point, our future will become more frightening, and we will lose our freedoms," a 22-year-old protester at the airport told AFP on Monday.

China warns of 'terrorism'

The movement has infuriated Beijing, where authorities Monday slammed violent protesters who threw petrol bombs at police officers, linking them to "terrorism".

"Hong Kong's radical demonstrators have repeatedly used extremely dangerous tools to attack police officers, which already constitutes a serious violent crime, and also shows the first signs of terrorism emerging," said Yang Guang, spokesman for the Hong Kong and Macao Affairs Office of the State Council.

"This wantonly tramples on Hong Kong's rule of law and social order."

Hours later, two state media outlets ran videos showing armoured personnel and troop carriers purportedly driving to Shenzhen, which borders Hong Kong.

And a commentary early Tuesday on China's official Xinhua news agency spoke of "black-clad mobsters" and said Hong Kong's future is at a "critical juncture".

A senior official in the administration of US President Donald Trump on Monday urged "all sides" to avoid violence in Hong Kong.

"Societies are best served when diverse political views are respected and can be freely and peacefully expressed," the official said on condition of anonymity.