Protesters clash with police outside Hong Kong Polytechnic University. A police officer was shot in the leg with an arrow and an armoured vehicle was set on fire, as protesters continue to defend their makeshift base at the Polytechnic University.

Hong Kong police laid siege to a university on Monday, firing rubber bullets and tear gas to pin back anti-government protesters armed with petrol bombs and other homemade weapons from fleeing amid fears of a bloody crackdown.

Dozens, choking on the tear gas, tried to leave the Polytechnic University by breaking through police lines after a night of mayhem in the Chinese-ruled city in which roads were blocked and a bridge was set on fire and a police officer was shot by a bow and arrow.

Many protesters, dressed in regular clothes and without gas masks, made runs for it, dodging tear gas canisters and sponge grenades, only to be forced back inside.

Some were arrested, tackled to the ground, as others scrambled and tripped over barricades and fences as police pointed guns at them and threw punches.

"The police might not storm the campus but it seems like they are trying to catch people as they attempt to run," Democratic lawmaker Hui Chi-fung told Reuters.

"It's not optimistic now. They might all be arrested on campus. Lawmakers and school management are trying to liaise with the police but failed."

Police were putting up barricades of their own to keep the protesters in one place. Earlier, police urged the protesters to "drop their weapons" and leave.

Police did not explain their tactics on Monday.

"We've been trapped here for too long. We need all Hong Kongers to know we need help," said Dan, a 19-year-old protester on the campus, as he burst into tears.

"I don't know how much longer we can go on like this. We may need international help."