Fewer than 100 protesters remain holed up in Hong Kong's Polytechnic University as riot police encircle the campus, with some activists desperately searching for ways to escape while others hid.

Some protesters told Reuters they were holding out not for a showdown with police, but because they were innocent and looking for an escape route.

"I won't consider surrendering. Surrendering is for people who are guilty. None of us inside are guilty," Michelle, a 20-year-old student, said.

Rubbish has piled up around the campus, with trash and debris from homemade petrol bombs littered across the grounds. Many protesters have abandoned their equipment, including gasmasks and umbrellas.

Much of the campus is damaged, with rooms vandalised and windows shattered. Electricity and water are still functioning.

The Asian financial hub has had a brief respite from months of often violent demonstrations, with relative calm across the city for the past two days and nights ahead of District Council elections set for Sunday.

The government said late yesterday it was closely monitoring the situation to see whether the elections could be held safely.

The university, in the centre of the bustling Kowloon peninsula, is the last campus still occupied by activists during a week that saw the most intense violence since the anti-government demonstrations escalated more than five months ago.

Demonstrators are angry at what they see as Chinese meddling in freedoms promised to Hong Kong when the British colony returned to Chinese rule in 1997.

Students trapped inside the university issue an S.O.S for help

Beijing has said it is committed to the "one country, two systems" formula granting Hong Kong autonomy.

The unrest marks the most serious popular challenge to Chinese President Xi Jinping since he came to power in 2012.

Some protesters have surrendered while others were held during escape attempts that included clambering down from a bridge to waiting motorbikes and fleeing through the sewers.

Graffiti sprayed on campus buildings read: "I have nothing to lose. I have no stake in society," summing up the mood felt by many of the protesters this morning.

Some looked for breakfast in one of the university canteens, which remained stocked with food, including noodles and tomatoes.

One protester, dressed in black clothes with gloves, elbow and knee pads, had about a dozen colourful lighters strapped to his chest, apparently to light petrol bombs. He told Reuters those inside were discussing what to do next.

In the past two weeks, protesters have torched buildings and infrastructure, including a footbridge, mass transit stations and toll booths at the city's Cross Harbour Tunnel linking Hong Kong island to the Kowloon peninsula.

The protesters say that they are angry at the way the MTR, Hong Kong's public rail network, has helped riot police, and that shutting down key infrastructure forces the government to listen to their demands for universal suffrage and an independent inquiry into police violence, among other things.

The Cross Harbour Tunnel remains shut because of extensive damage, authorities said.

Some train services remained shut, the city's metro operator MTR Corp said, while the rural Yuen Long station in New Territories would shut by 2 pm, to pre-empt demonstrations marking four months since suspected triad gang members attacked protesters and commuters there.