Police have besieged a university campus in Hong Kong occupied by protesters who have been fighting back with arrows and petrol bombs.
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.
"I hereby warn rioters not to use petrol bombs, arrows, cars or any deadly weapons to attack police officers," police spokesman Louis Lau said in a statement broadcast via Facebook.
"If they continue such dangerous actions, we would have no choice but to use the minimum force necessary, including live rounds, to fire back."
Earlier on Sunday police fired a live round in response to what they said was a car hurtling towards officers near the university.
Determined to fight to the end
By the BBC's Gabriel Gatehouse in Polytechnic University, at 04:00 Hong Kong time
It's only the most radical left now – or the bravest, depending on your point of view. About a hundred of them are hunkered down at a junction outside the main entrance. On the other side of the road: an armoured police vehicle, and a water cannon truck.
Every 10 minutes or so, these two sides play a game of cat and mouse. The police fire tear gas and the water cannon advances, squirting noxious blue liquid. The protesters, crouching behind umbrellas, respond with petrol bombs and rocks fire from improvised catapults. The police vehicles retreat. The net result is zero. It's a stalemate. There are several hundred more protesters milling around the campus. Medics treat those who have been hit by tear gas or the blue liquid in the water cannon, which stings on contact with the skin. Others man barricades at the many entrances to this sprawling campus which is now completely surrounded. When the police announced a 22:00 deadline for the protesters to surrender or face the possibility of lethal force a significant number changed out of their black outfits, ditched their masks, and disappeared into the night. Some were arrested but others escaped.
Those who remain seem determined to fight to the end, no matter the risk. "If I die, remember me," one young man said to me. "Do you believe that could happen?" I asked. He gave a nervous shrug. But the promised police assault has, so far, not materialised.
The protests in Hong Kong, which began in June, were triggered by a now-withdrawn plan to allow extradition to mainland China but have since expanded into wider demands for greater democracy and for investigations into the actions of police.
The government recently confirmed the city had entered its first recession for a decade.
In recent days, Hong Kong's university campuses have been the scenes of pitched battles between police and demonstrators.
Protests in Hong Kong
On Sunday, riot police fired tear gas and used water cannon against protesters at the PolyU, who launched bricks and petrol bombs at them. Protesters took cover behind umbrellas on a footbridge and set light to debris there, causing a huge fire.
The blaze triggered a number of small explosions, witnesses said, and fire crews eventually moved in to douse the flames.
There were also heavy clashes on a bridge above the Cross Harbour tunnel, which links Kowloon and Hong Kong island. A police truck on the bridge was set on fire and forced to retreat.