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 earlier wounded in the leg with an arrow near the Polytechnic University (PolyU).
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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.
Police have so far been responding to violence around the PolyU campus mostly with tear gas, rubber bullets and water cannon.
Those occupying the university have been told to leave immediately. Dozens have reportedly been arrested but hundreds remain inside.
There are fears of bloodshed should police move in to quell what they have now declared a riot.
Earlier on Sunday police fired a live round in response to what they said was a car driving towards officers near the university.
The protests in Hong Kong, which began in June, were triggered by a now-withdrawn plan to allow extradition to mainland China but have since expanded into wider demands for greater democracy and for investigations into the actions of police.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.
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 have also been 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.
