By Donny Kwok and Clare Baldwin

HONG KONG, Nov 21 (Reuters) - Fewer than 100 protesters remained holed up in a Hong Kong university on Thursday as riot police encircled the campus, with some activists still desperately searching for ways to escape while others hid.

Some protesters told Reuters they where 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 on the campus of Polytechnic University on the Kowloon peninsula.

Rubbish has piled up around the campus, with mountains of trash and debris from homemade petrol bombs littered across the grounds. Many protesters have abandoned their belongings, including gas masks 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 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 on Wednesday night 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.

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 the society," summing up the mood felt by many of the protesters on Thursday 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. He told Reuters the remaining protesters were discussing what to do next.