Hong Kong police have confirmed that a police officer “disguised” as a protester fired a single warning shot on a street in Hong Kong Island’s Wan Chai district on Sunday evening amid an unauthorized march that led to violent clashes.

At about 5.30 pm, police officers on duty near Wan Chai MTR Station “were surrounded and attacked by a large group of violent protestors,” according to a government statement issued at 1 am on Monday.

“With their lives under serious threat, an officer fired one warning shot into the sky to protect their own safety.”

However, the statement did not mention that the officers were working undercover.

Video footage of the incident later appeared on social media. It shows a group of masked men – dressed liked protesters and brandishing batons and umbrellas – chasing protesters near the Southern Playground on Johnston Road.

It then shows some protesters fighting back with wooden sticks and umbrellas, and surrounding several suspected undercover officers.

Then a gunshot is heard. Protesters shout “gunshot” and flee. A bullet casing was later found on the ground.

The suspected undercover police officers then ran into the changing room of the playground and were escorted by other uniformed police officers to the Wan Chai police station, according to a passer-by who witnessed the incident from the playground podium, Radio Free Asia reported.

During the police press conference on Monday, Senior Superintendent Li Kwai-wah of the Organized Crime and Triad Bureau admitted the officers were “disguised” as protesters but he would not disclose details regarding the number of officers deployed or how many of them carried guns, insisting it would not be appropriate to disclose such information.

Senior Superintendent Kong Wing-cheung of the Police Public Relations Branch, meanwhile, dismissed an allegation that undercover officers set a fire outside Wan Chai MTR station in order to frame protesters for arson.

Hong Kong Island turned into a battleground on Sunday after protesters gathered in Causeway Bay for a banned event called the “Global Anti-totalitarian March.” But police were seen taking an unusual early action – they fired rounds of tear gas outside Sogo department store at 2:30 pm, when some of their fellow officers had not yet put on their gas masks and the street was filled with shoppers.

The protesters were pushed back and then marched toward Admiralty. They regrouped outside the government headquarters and then threw bricks and Molotov cocktails at police.

The force responded with even more tear gas, backed up by their water-cannon-equipped anti-riot vehicle, which sprayed protesters near the government headquarters at Tamar.

The “raptor” squad launched rapid strikes against the protesters in Admiralty, and dozens were arrested.

Officers hit protesters with their batons and some were seen bleeding heavily.

A total of 146 people were arrested on Sunday, including 118 males and 28 females aged between 12 and 53, for alleged unlawful assembly and possession of weapons. Sixty-seven of them were students.

The police fired 328 tear gas canisters, 306 round of rubber bullets, 95 bean-bag rounds, and 70 sponge bullets. Eight police officers were injured.