Hong Kong police fired tear gas and rubber bullets at protesters holed up inside one of the city's universities, after a night of mayhem in which a police van was set alight and a police officer was shot with an arrow.

Key points: Hong Kong's high court ruled a ban on facemasks was "unconstitutional"

Hong Kong's high court ruled a ban on facemasks was "unconstitutional" Protesters were met with tear gas as they attempted to flee

Protesters were met with tear gas as they attempted to flee Police had tried to storm the campus of Hong Kong Polytechnic University overnight

The siege of Polytechnic University dragged into a second day as the city's High Court delivered a victory to pro-democracy protesters, striking down a facemask ban imposed by the government last month.

Groups of masked protesters who tried to break out of the campus on Monday morning were quickly met by rounds of tear gas fired by police at several entrances. Some managed to escape, while dozens were arrested by police, local media reported.

Demonstrators then set the main campus entrance ablaze after police warned they may use live rounds, deepening fears over how nearly six months of unrest across the city will end.

Demonstrators set Hong Kong Polytechnic University's main entrance ablaze to prevent surrounding police moving in. ( AFP: Ye Aung Thu )

Amid some of the most violent clashes in the city's months-long disturbances, the court said it did not consider anti-mask laws unconstitutional in general, but in this case, the law infringed on fundamental rights further than was reasonably necessary.

Many protesters wear masks to shield their identities from surveillance cameras that could be used to arrest and prosecute them.

The ban has been widely ignored, and police have charged protesters with wearing masks.

Police warned they were ready to use live bullets if protesters continued to use lethal weapons. ( Reuters: Tyrone Siu )

Meanwhile, other activists stopped traffic near the university and forced shopping malls and stores to shut.

Witnesses saw some protesters suffer burns from chemicals in the jets fired from police water cannons, according to Reuters.

The United States condemned the "unjustified use of force" in Hong Kong and called on Beijing to protect Hong Kong's freedom, a senior official in President Donald Trump's administration said.

"We condemn the unjustified use of force and urge all sides to refrain from violence and engage in constructive dialogue," the official said.

'All hell is breaking loose'

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 17 seconds 1 m 17 s Violent scenes outside Hong Kong Polytechnic University.

On Sunday night, police tried to enter the university but were forced back by protesters who set huge fires.

ABC China correspondent Bill Birtles was at the scene and described what he saw as "all hell is breaking loose".

"The police Raptors — the special tactical forces — I saw them race past the no-man's land through the protest line, and they're firing a tremendous amount of tear gas," he told ABC Radio National.

"Police earlier in the night said they may resort to live rounds if protesters continued to throw fire bombs at them, and so this is what everybody is really concerned about."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 35 seconds 2 m 35 s Bill Birtles saw police storming the entrance to the university in a "tremendously brutal" operation on Sunday night.

After the clashes, Hong Kong Polytechnic University president Jin-Guang Teng said police would allow protesters to leave the campus, and that he would accompany them to the police station to ensure their cases, "will be fairly processed".

He said in a recorded video message that he hoped protesters would "accept the proposed temporary suspension of force and leave the campus in a peaceful manner".

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 9 seconds 1 m 9 s Hong Kong Polytechnic University's president claimed police would allow protesters to leave his campus.

It seemed unlikely the protesters would accept the offer, given they would all likely be arrested.

However a few hundred streamed out of the campus at about 8:15am (local time), only to be driven back by police tear gas.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 42 seconds 42 s Police lobbed tear gas at protesters attempting to leave the university.

Others, who were wearing gas masks and picked up the smoking tear gas canisters to drop them into heavy-duty bags, retreated in the face of officers who had lined up across the road in the distance.

These latest clashes follow an incident where a Hong Kong police officer was admitted to hospital after being shot in the leg by an arrow on Sunday afternoon.

Police action breaks a week-long demonstration

The Polytechnic University protesters had been blocking one of Hong Kong's major highways, the Cross Harbour Tunnel linking Hong Kong island to the Kowloon peninsula, for much of the past week.

Escalating violence between anti-government protesters and police in the former British colony has been condemned by Beijing and the city's Beijing-backed leaders.

In a statement, police warned rioters to stop using lethal weapons to attack officers and to halt other acts of violence, saying officers would respond with force and possibly live bullets if necessary.

Loading

The spectre of a bloodier stand-off has caused some international concern.

Former British foreign secretary Malcolm Rifkind said in a statement: "Hong Kong's Chief Executive has the responsibility to do everything possible to prevent a massacre. She must order the police to use restraint."

'We are fighting for Hong Kong'

Protesters are seen using a makeshift catapult during unrest in Hong Kong. ( ABC News: Steve Wang )

Hong Kong's protests have been running for months, triggered after the Government attempted to push through a controversial extradition bill that would have allowed China to detain dissidents in Hong Kong and try them in mainland courts.

While the bill has since been canned, the protests have focused on Beijing's alleged interference in the city's partial autonomy — guaranteed by China for a period of 50 years after Britain handed back the territory in 1997.

Multiple fires erupted on the university's perimeter. ( ABC News: Jake Lapham )

This autonomy gives Hongkongers capitalism, common law, and personal freedoms not found in mainland China.

On Monday, protesters continued pressing for other formal demands to the city's Government, which includes the retraction of the word "riot" to describe rallies, the release of all detained demonstrators, an independent inquiry into perceived police brutality, and universal suffrage.

"The protesters have been reacting to the police," said Joris, 23, a civil engineer who like others did not give his full name.

"We haven't fought back as much as we could. I would be prepared for jail. We are fighting for Hong Kong."

Beijing denies interfering in Hong Kong's affairs and has blamed foreign influences for the unrest.

Loading

ABC/wires