Hong Kong police fired tear gas at protesters as they tried to escape a besieged university on Monday while hundreds more pro-democracy activists stayed inside armed with petrol bombs and other weapons awaiting an expected operation to oust them.

The bloody standoff between security forces and activists at Polytechnic University entered its second day after a night of mayhem in which a police van was set alight and a police officer was shot with an arrow.

Thirty eight people were injured overnight Sunday, the city's Hospital Authority said. Reuters witnesses saw some protesters suffer burns from chemicals in the jets fired from police water cannons.

"Remember you have life in your hands. Why do you need to push us to death?" one person shouted at police from a campus rooftop as protesters wearing gas masks and clutching umbrellas looked for ways to escape the university.

Police arrested dozens of protesters near the university on Monday morning, public broadcaster RTHK reported.