Sunday's violence in the district of Tsuen Wan, around 10 kilometres (six miles) from the city centre, was some of the worst in 12 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.
An officer fell to the ground under a barrage of blows from "rioters" who had "the clear intention to take his life," Assistant Chief of Police Mak Chin-ho told reporters Monday.
"One officer fired a warning shot into the air," he added, while six officers held up their revolvers as a precautionary measure.
It is believed to be the first live round fired by an officer during the current crisis.
Twenty-one officers were injured during the clashes, Hong Kong police said, while dozens of protesters were arrested -- including a 12-year-old -- for unlawful assembly, possession of weapons and assaulting police.
Police have appealed to members of the public "to make a clean break" with violent protesters, vowing "relentless action" to bring the perpetrators to justice.
The firing of a live round sparked anger on social media, with some mocking a police spokesman who had praised the "valiant and restrained" actions of officers on Sunday.
The violence erupted after a nearby peaceful march earlier Sunday. Police used tear gas against the black-clad hardcore protesters, who had built barricades and thrown 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 antagonism between the movement and the police is too big to fix now," said a 20-year-old protester who was at Sunday's demonstration.
- Deadlock -
The weekend of violence -- unrest also flickered across the semi-autonomous Chinese city on Saturday -- banished several days of peaceful action, which included a human chain across Hong Kong.
With almost daily protests planned throughout the week and another major rally expected on Saturday, the movement shows no sign of abating.
The protests were ignited when the city's Beijing-backed government tried to pass a bill allowing extraditions to mainland China, but has evolved into a wider call for greater democracy and police accountability.
The mainly young protesters -- whose angst also stems from the sky-high cost of living and a lack of jobs -- say the future of the city of around seven million people hangs in the balance with Beijing chiselling away at their freedoms.
In France on Monday, the leaders of the G7 nations -- including Prime Minister Boris Johnson of Britain, which handed Hong Kong over to China in 1997 -- urged Beijing to respect the city's autonomy.
"The G7 nations all want to support a stable and prosperous Hong Kong and we remain collectively committed to the one-country, two-systems framework," Johnson said.
But protesters have shown few signs of leaving the streets, while the city government has also refused to give ground.
Last week, the city's embattled leader Carrie Lam made vague promises of opening a "dialogue" to find a way through the crisis.
She met Saturday with grandees from politics and education.
"But can we point to a person who can honestly, genuinely be regarded as representing the protesters in the streets? No. Not a single one," pan-democrat lawmaker James To Kun-sun, who supports the movement, told reporters Monday.
The deadlock has sunk Hong Kong deep into crisis.
Beijing has used a combination of intimidation, propaganda and economic muscle to stifle the protests in a strategy dubbed "white terror" by the movement.
Major Hong Kong businesses have come under pressure over staff support for the demonstrations, most notably airline Cathay Pacific and the city metro system -- the MTR -- which has been accused of ferrying demonstrators to rally sites.
Since the allegations surfaced in Chinese state media, the MTR has closed stations near planned protests.
The protests are hacking away at Hong Kong's image as a prosperous and stable financial hub. (AFP)