Hong Kong police have stormed a university campus held by protesters after an all-night siege that included firing repeated barrages of tear gas and water cannons.

The dramatic escalation of violence came after anti-government protesters had barricaded themselves inside Hong Kong Polytechnic University for days.

Police surrounded the area on Sunday night and began moving in after issuing an ultimatum for people to leave the area. The crowd wore raincoats and carried umbrellas to shield themselves.

ABC China correspondent Bill Birtles, who was on the ground near Hong Kong Polytechnic University early on Monday, said “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.

He said protesters were pushing back, including with petrol bombs.

Morning has broken in #HongKong at Poly U and it’s an awful start – at least hundreds of students are inside that campus… which is on fire. Small explosions every minute. The middle of the campus is quite open so hopefully students can take refuge there pic.twitter.com/4wb95bRUh4

— Bill Birtles (@billbirtles) November 18, 2019

As Monday wore on, the siege showed no sign of ending. There were thought to be hundreds of students holed up in the campus, with thousands more expected to come to their aid throughout Monday.

“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,” Birtles said.

Riot officers broke in one campus entrance before dawn on Monday but didn’t appear to get far, being met with walls of fire inside and out.

There were also fiery explosions as protesters lobbed petrol bombs at officers. Police, who had warned that everyone in the area could be charged with rioting, reportedly made a handful of arrests.

Around 100 people arrested earlier in Tsim Sha Tsui have their hands zip-tied by police and are made to sit on the ground

Video: SCMP/Alvin Lum pic.twitter.com/gceec2PlwI

— SCMP Hong Kong (@SCMPHongKong) November 18, 2019

At daybreak on Monday, protesters remained in control of most of the campus.

In one outdoor area, some demonstrators made petrol bombs while others dozed while wearing gas masks. Two carried bows and quivers of arrows.

The university president said in a video message that police had agreed to suspend their use of force.

Jin-Guang Teng said police would allow protesters to leave and he would accompany them to the police station to ensure their cases would be processed fairly.

“I hope that you will accept the proposed temporary suspension of force and leave the campus in a peaceful manner,” he said.

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

A few hundred streamed out of the campus about 8.15am in an apparent bid to escape, but they were driven back by police tear gas.

In #HongKong, the police have started firing “rounds and rounds” of tear gas at the #HongKongProtesters, trapping them inside #PolyUHK. #China’s brutal crackdown has turned #HK into a war zone.

China’s strategy: censor & intimidate at all costs.
pic.twitter.com/JektZXDKo6

— Steve Hanke (@steve_hanke) November 18, 2019

Some wearing gas masks calmly picked up smoking tear gas canisters and dropped them into heavy-duty bags. Then, with a phalanx of officers lined up across the road in the distance, they decided to retreat .

Earlier, Polytechnic University said “dangerous chemicals” were stolen from laboratories and the campus had been “been widely damaged.” by activists’ “illegal acts and violence”.

“We understand that students care about the current social situation, however, they must be calm and rational when fighting for anything,” the university said in a statement.

“Resorting to violence or other radical acts will not help solve the problem.”

Junior Police Officers’ Association chair Lam Chi-wai also said officers’ lives had been threatened by “deadly weapons”. He called the protesters “rioters who have lost their rational minds”.

“When a rioter raises a petrol bomb to prepare to throw it, police officers on scene may very likely see it as a deadly attack upon them or others, and use relevant force or a weapon to stop it, including live ammunition,” Mr Lam said.

Protestors throw petrol bombs at an armoured police vehicle.

In a live Facebook broadcast, police spokesman Louis Lau officially warned the demonstrators.

“Stop using petrol bombs, arrows, vehicles or any other lethal weapons to attack police officers, and stop all acts of assault,” he said.

“If they continue these dangerous acts, we will have no choice but to use the necessary minimal force, including live ammunition, to hit back.”

On Sunday, protesters used bows and arrows, and one arrow struck a media liaison officer in the calf. Photos on the department’s Facebook page show the arrow sticking out of the back of the officer’s leg.

As riot police moved in from all sides, some protesters retreated inside the university. Others set fires on bridges leading to it.

A huge blaze burned along much of a long footbridge that connects a train station to the campus over the approach to the Cross-Harbour Tunnel, a major road under Hong Kong’s harbour that has been blocked by protesters for days.

The protests started peacefully in early June, sparked by proposed legislation that would have criminal suspects to be extradited to the mainland.

But by the time the bill was withdrawn, the protests had broadened into a resistance movement against the territory’s government and Beijing.

The protesters held their ground for most of Sunday, as water cannon trucks drove over bricks and nails they had strewn on the ground. Some were sprayed at close range with water dyed blue to help police identify perpetrators.

The activists retreated and barricaded the entrances to the campus and set up narrow access control points.

Australia has expressed deep concern about escalating violence in Hong Kong following the storming of the university campus.

“We are very concerned about the scenes that we have seen today,” Foreign Minister Marise Payne said in Sydney on Monday.

The US has also condemned the “unjustified use of force” and called on Beijing to protect the city’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 senior US official said.

“As the President has said, the United States expects Beijing to honour its commitments under the Sino-British joint Declaration and to protect Hong Kong’s freedom, legal system and democratic way of life.”

-with AAP