HONG KONG - Hong Kong police said Monday they were forced to fire water cannon and a warning shot to fend off "extremely violent" demonstrators, following another weekend of clashes at pro-democracy rallies.

Sunday's violence in the district of Tsuen Wan, around 10 kilometers from the city center, was some of the worst in twelve weeks of political unrest roiling the international financial hub.

As night fell, a group of officers were cornered by protesters armed with bricks and other weapons, Hong Kong police said in a statement.

When one officer fell to the ground under a barrage of blows, and "facing threats to life", six others drew their sidearms and one "fired a warning shot into the sky", it said.

It is believed to be the first live round fired by an officer during the current crisis.

Fifteen officers were injured during the clashes, the statement said, while dozens of protesters were arrested -- including a 12-year-old -- for unlawful assembly, possession of weapons and assaulting police.

"Police appeal to members of the public to make a clean break with violent protestors," the statement added, vowing "relentless action" to bring the perpetrators to justice.

The firing of a live round prodded an angry response from social media users, who mocked a police spokesman who had praised the "valiant and restrained" actions of officers on Sunday.

"If the police can't control their emotion, how can they be valiant and restrained?" a Facebook user asked.

The violence erupted after a peaceful march nearby earlier Sunday. Police used tear gas against the black-clad, gas mask-wearing hardcore protesters, who had built barricades and thrown bricks and Molotov cocktails at lines of riot cops.

Police confirmed they deployed two water cannon vehicles "to stop violent acts". The jets hit the barricades as demonstrators ran for cover.

The weekend of violence -- unrest also flickered across the city on Saturday -- banished several days of peaceful action, which included a human chain across the city.

With almost daily protests planned throughout the week and another major rally expected on Saturday, the movement shows no sign of abating. -AFP