A heavily-fortified university campus in Hong Kong has been stormed by police with some declaring it a “war zone”.

Police stormed the Hong Kong Polytechnic University before dawn on Monday where anti-government protesters had barricaded themselves behind sophisticated and elaborate walls.

They had been fighting back using catapults, petrol bombs and bows and arrows — one of which landed in the leg of a police officer.

The ABC’s China correspondent, Bill Birtles, is in the thick of it and says “all hell is breaking loose”.

In a series of tweets from the ground, Birtles wrote that police had blocked all roads and exits around the university and arrested those who were trying to administer first aid.

“The saddest photo of the night,” he wrote alongside a picture of volunteers with their wrists tied together behind their backs.

In video footage from the bridge where protesters have gathered, a police truck attempts to drive through their barricade. But it quickly changes course after being hit by petrol bombs.

Early reports were that riot police had begun firing tear gas — and pictures back that up. But some reports suggest police have resorted to firing live rounds at protesters who have set fire to bridges leading to the campus.

#LIVE: Some protesters at @HongKongPolyU hop out from the fence and try to escape. But police fire tear gas at them apparently to prevent them from leaving. Some citizens outside the police cordon cry and beg the police to let them leave: “They are just kids!” #HongKongProtests pic.twitter.com/rRY8uWmBJJ — Ezra Cheung (@ezracheungtoto) November 18, 2019

The scenes today — loud explosions and walls of fire — could not be more different to the calm before the storm in footage of the fortified campus shared by local media earlier this week.

One video, from the South China Morning Post, shows what police would be confronted with if and when they decided to make their move. That move came in the early hours while many protesters slept.

Following fiery stand-offs with Hong Kong police, student protesters fortified defences around their university campuses, shutting down streets, motorways and rail lines. https://t.co/pZTgwUztC9 #HongKongProtests pic.twitter.com/z2zlZasTGu — SCMP News (@SCMPNews) November 15, 2019

Now it’s a war zone. The numbers are hard to get a grip on but it’s believed hundreds of student protesters are trapped at the university but thousands of others are coming to their aid.

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 the protesters for days.

Smoke can be seen drifting across the Hong Kong skyline.

The use of bows and arrows, along with gasoline bombs launched with catapults, threatened to escalate the violence in the more than five-month-long anti-government movement. Protesters are trying to keep the pressure on Hong Kong leaders who have rejected most of their demands.

The protests were sparked by proposed legislation that would have allowed the extradition of criminal suspects to the mainland.

Activists saw it as an erosion of Hong Kong’s autonomy under the “one country, two systems” formula implemented in 1997 when Britain returned the territory to China.

The bill has been withdrawn, but the protests have expanded into a wider resistance movement against what is perceived as the growing control of Hong Kong by Communist China, along with calls for full democracy for the territory.

Several hundred people formed a human chain on Sunday in central Hong Kong in a peaceful rally in support of the movement.

Azaze Chung, a university student, said the Government should respond to the protesters’ demands, not just use force against them.

Police and protesters faced off all day outside the uni after a pitched battle the previous night in which the two sides exchanged tear gas and gasoline bombs that left fires blazing in the street.

A large group of people arrived in the morning to try to clean up the road but were warned away by protesters.

Riot police shot several volleys of tear gas at the protesters who sheltered behind a wall of umbrellas and threw gasoline bombs into nearby bushes and trees, setting them on fire.

The protesters held their ground for most of the day, as water cannon trucks drove over bricks and nails strewn by protesters to spray them at close range – some with water dyed blue to help police identify protesters afterwards.

Protesters began retreating into the university near sunset, fearing they would be trapped as police fired tear gas volleys and approached from other directions.

The protesters have barricaded the entrances to the campus and set up narrow access control points.

They are the holdouts from larger groups that occupied several major campuses for much of last week.

Another group threw bricks in the street to block a main thoroughfare in the Mongkok district, as police fired tear gas to try to disperse them. The disruption to Nathan Road traffic may have been an attempt to distract police during the standoff at Polytechnic.

Opposition politicians criticised the Chinese military for joining a clean-up to remove debris from streets near Hong Kong Baptist University on Saturday. Dozens of Chinese troops, dressed in black shorts and olive drab T-shirts, ran out in loose formation and picked up paving stones, rocks and other obstacles that had cluttered the street The military is allowed to help maintain public order but only at the request of the Hong Kong Government.

The Government said it had not requested the military’s assistance, describing it as a voluntary community activity. The Education Bureau announced that classes from kindergarten to high school would be suspended again on Monday because of safety concerns. Classes have been cancelled since Thursday after the bureau came under criticism for not doing so earlier.

Associated Press video journalist Dake Kang contributed to this story.