A Hong Kong police officer fired a gunshot during protests on Sunday, the first time a live round has been used since demonstrations broke out in June.
Images show several other officers also pointed guns at protesters who were charging at them with sticks and poles.
In another first, police deployed water cannon against protesters earlier in the day.
The protests began in the city’s Tsuen Wan district and then spread to the district of Tsim Sha Tsui.
Hong Kong police said 36 people had been arrested as a result of Sunday’s protests, the youngest aged just 12
Demonstrations were sparked by an extradition bill but have since morphed into broader anti-government protests, and developments on Sunday marked a serious escalation in the unrest.
Police told local media that the gunshot was fired as a warning to protesters, and that several officers had been taken to hospital as a result of the clashes.
In a statement on Monday they said six officers had drawn their pistols and one had fired the warning shot into the air.
Police Superintendent Leung Kwok-wing did not say where the shot was aimed but told news outlet SCMP that the shot was fired because the lives of police officers were “under threat”.
During the melee, one older man was seen on his knees pleading with police not to shoot.
Before the scuffle, black-clad protesters had thrown projectiles at police, including bricks and petrol bombs.
Police also fired rubber bullets and tear gas, and used two water cannon vehicles to clear barricades and disperse crowds.
Images on social media showed the vehicles being driven through the streets of Tsuen Wan, where a group of demonstrators had set up roadblocks and dug up bricks from the pavement.
The vehicles are equipped with surveillance cameras and multiple water cannon nozzles, and police had said they would only be used in the event of a “large-scale public disturbance”.
Earlier this month, human rights group Amnesty International warned that the cannons could cause serious injuries and lead to further tensions.
Activists have criticized the police for being heavy-handed during the pro-democracy rallies, which saw several people hospitalized after violent clashes on Saturday.
The latest round of violence followed a stretch of uneasy peace in Hong Kong. But on Saturday, a senior Hong Kong official warned that the mainland’s military could intervene during any civil unrest.
Maria Tam Wai-chu, deputy director of the Hong Kong SAR Basic Law Committee, told a seminar in southern China: “The soldiers stationed in Hong Kong are not straw men meant to just stay in the garrison, they are an important part of the ‘one country, two systems'”.
On Sunday another more peaceful rally was held comprising a few hundred people, some of them family members of the police, who called for a political solution to the crisis.