The protesters had to lower themselves about 10 metres onto the road below, and were met with a hail of projectiles fired by police as they descended. They were sped away by a convoy of rescuers on motorbikes. Some of the motorcycles were waiting for them, while others sped to the scene.

Not everyone seems to have got away. Some of the motorbikes were later seen abandoned on the road and a number of protesters appeared to have been arrested.

Earlier, a group of protesters who rushed out of the campus were tackled to the ground and subdued by riot police who also raised weapons at them. Police continued to fire tear gas into the campus.

Police said 400 people had been arrested at the Polytechnic campus.

The High Court, meanwhile, struck down a controversial mask ban that had been introduced under emergency law and police said they would stop making arrests of people in masks.

Parents and school principals urged police to allow the hundreds of student protesters still holed up inside the Polytechnic university on Monday afternoon to leave safely, but police declared the campus had become a "battlefield for criminals and rioters".

"Surrender is the only viable option," said a police spokesman. Yet tear gas was fired at students trying to leave.

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The vice chancellor of the university Teng Jin-guang told police he wanted to accompany arrested students to the police station.

Security minister John Lee said protesters in the campus should “leave as soon as possible” as chemicals inside risked exploding.

The government would send a team of social workers and psychologists inside the campus to locate children and accompany them out to bring them to the police station.

He said if protesters surrendered at points designated by the police they could come out peacefully.

A group of 20 protesters who fled Polytechnic along train tracks were rounded up by police and put on a train. Social workers were also arrested when they tried to leave.

Riot police clashed with protesters in multiple locations across Hong Kong on Monday as protesters reacted angrily to the scenes at Polytechnic. The government warned District Council elections due in six days may be postponed if the violent scenes continued.

The protesters at Polytechnic had barricaded themselves inside the campus at the weekend, stockpiling weapons in preparation for a police intervention.

The police moved on Sunday, and retook the roads on three sides of the university, but faced fierce resistance from protesters who hurled petrol bombs at a water cannon and armoured vehicle.

Riot police moved in before dawn firing rubber bullets and tear gas to clear protesters off the road and make arrests. Students fled inside the heavily fortified campus. Police said they had made arrests near the entrances but then withdrew.

Police had earlier warned they may use live bullets.

Booms and cracks filled the air as petrol bombs and tear gas canisters exploded and set fire to trees when police moved in before dawn on Monday morning. Fire quickly spread up the staircase of the Polytechnic University's main entrance.

Police tackled protesters to the ground on the road outside the campus and dragged them away. But more than 500 protesters remained in the multi-storey Polytechnic building on Monday.

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Protesters trying to leave had tear gas fired at them, despite a police call for them to come out of the fortress-like building with their masks off.

At least one pro-democracy politician was arrested trying to enter the Polytechnic campus, which police seemed determined to isolate.

"I didn't sleep last night. I watched the live footage from midnight until 6am, then I caught the first bus here," said Jane, 26, in tears and among a large group of people that gathered in a nearby square at Tsim Sha Tsui - outside of the police cordon.

She was waiting to hear news of her friends inside the Polytechnic campus.

A violent confrontation

Throughout the siege, protesters with umbrellas and petrol bombs had held back the police water cannon, which repeatedly sprayed liquid laced with pepper spray, on a road in front of a People's Liberation Army barracks.

But protesters appeared caught by surprise when riot police stormed the fortified university campus before dawn on Monday, rushing behind protester lines and firing tear gas and rubber bullets from rifles.

But when they finally charged the riot police came from a different direction and out of range of a catapult students had deployed inside the university.

After the attack the police retreated but a large fire appeared to rage inside the university building where many protesters had fled.

As the road outside the university sat empty after the special forces police withdrew, explosions could be heard and seen inside the university building where protesters remained.

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Protesters had piled flammable material and gas and petrol bombs inside as an arsenal.

After daybreak, rubber bullet casings and spent gas canisters lay on the road in front of the university entrance.

A pair of goggles sat in a pool of blood where they had been abandoned.

Police had been seen dragging protesters away from the university after grabbing them near its entrance.

Many of the volunteer medics who would usually assist injured protesters had been arrested by police earlier as they tried to leave the campus.

The fires and police action followed a tense night at the university, when police used a live Facebook broadcast to warn that officers would fire live ammunition if protesters did not stop targeting police.

A single live round was fired at 10.20pm near China's People's Liberation Army barracks when a vehicle did not stop, police said.

Chanting from the nearby Chinese barracks could be heard but the PLA stayed in its barracks.

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Earlier in the evening, a police armoured vehicle tried to drive through barricades on the road to the university that overlooks a major cross harbour tunnel.

Protesters threw petrol bombs at the vehicle, setting it on fire, and forcing it to retreat.

More than 50 people wearing media or medic vests were among those arrested leaving the Polytechnic area, police said.

With Reuters