Officers have warned that they could use live ammunition if protesters do not stop attacking them using such weapons.
A media liaison officer was on Sunday wounded in the leg with an arrow near the Polytechnic University (PolyU).
Months of anti-government protests have caused turmoil in the city.
The latest violence is however some of the worst the semi-autonomous Chinese territory has seen since the movement began. The police have become targets for radical demonstrators, who accuse them of excessive force.
Police have so far been responding to violence around the PolyU campus mostly with tear gas and water cannon.
Those occupying the university were told to leave immediately on Sunday evening. Dozens have reportedly been arrested but hundreds remained inside early on Monday. There are fears of bloodshed should police move in to forcefully quell what they have now declared a riot.