Police massed on a pedestrian footbridge a few hundred metres from the campus main entrance but were stopped as protesters set fire to barricades on the footbridge.

They began a renewed attack on the campus at 11.15pm after warning they would make mass arrests.

Earlier, police had instructed people inside the campus should leave via the northern exit at Lee Shau Kee Building and follow police orders.

But people – including journalist who did not have accreditation – were arrested as they left through the police checkpoint. First-aid volunteers and medics in orange vests who tried to leave the scene were also arrested in large numbers. They were made to line up in rows and sit on the ground with their hands cable-tied behind their backs.

Many protesters had stayed and began throwing petrol bombs as the police water cannon moved in again opposite the military barracks. Flames spread across the road.

Senior management of the university also issued a statement condemning violence and told students to leave.

For days students had blocked a major road and tunnel crossing by repeatedly bombing the toll booths behind the campus.

On Sunday after police moved in to try to clear blockades a police media officer was shot in the leg with an arrow 30 metres from the university, around 2pm.

Students on balconies used two large catapults to hurl rocks down on the police vehicles below, while at dusk a government helicopter buzzed overhead.

Barricades were on fire on footbridges - that had been occupied by protesters earlier - linking the university to the Hung Hom train station.

As social workers scouted urgently for safe exits from the multi-storey brick Polytechnic campus, black-clad protesters sat on the ground inside making gas and petrol bombs.

For hours student protesters and the police had clashed on the intersection between the Polytechnic university and a Chinese military barracks. Hundreds of student protesters crouched behind barricades on the street outside, from where they threw petrol bombs at police. Other students positioned on the rooftop used catapults to fire petrol bombs down on to the road at heavily armoured police vehicles.

Police used a water cannon to spray protesters with pepper spray-laced water as they crouched behind umbrellas in rain coats. For the first time a sound cannon was also deployed against protesters. Reporters hit with the water were shepherded away from the frontline by medics.

A volunteer with Save the Children, Pang Pang, 47, said if police came inside the campus there could be “a massacre if you ask me”.

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“I don’t have a crystal ball, right now the children and us are concerned about our dignity and freedoms - not our lives.”

Teachers' unions released an emergency statement urging police not to storm the campus where thousands of protesters were holed up.

Offices inside the building were choked with tear gas residue and water dripped onto the carpet from broken pipes.

On the ground floor a medical centre had been set up.

Police accused the protesters of endangering safety as “rioters continue to launch hard objects and petrol bombs with large catapults at police officers”.

A police statement said a “large amount of offensive weapons, including flammable fluids, are stored on the campus, jeopardising public safety”.

American pastor William Devlin said he had come as a human rights observer and would report back to the US State Department.

A large group of local church volunteers from Save the Children also gathered inside the campus.

Polytechnic University is across the road from the Chinese People's Liberation Army barracks, where soldiers observed proceedings through binoculars.

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Thousands of students have been barricaded inside the campus since Wednesday.

Police responded by spraying tear gas and blue dye from a water cannon and, for the first time, a sound cannon was also deployed against protesters.

Polytechnic's president Teng Jin-Guang sent an email to all students on Friday asking them to leave the campus, which he said had been overtaken by activists who had "severely and extensively vandalised" facilities.