Hong Kong police have been accused of using undercover officers disguised as protesters to make mass arrests during the anti-government street rallies.

Video footage posted online showed one demonstrator being wrestled to the ground by an officer in riot gear and a man wearing jeans, trainers, t-shirt and face mask.

The protester, who said his name was Chow Ka-lok, was left with blood pouring from his face and a broken tooth.

“I understand, don’t press me, I’m sorry,” he said as the suspected plainclothes officer pinned his face down using one knee. During the video the man assisting with the arrest told the protester “I’m on your side”, according to a report in the Hong Kong Free Press.

It was one of a series of arrests of protesters who were attempting to block major roads in Causeway Bay on Sunday night.

Activists said they were confronted by a group of 20 to 30 masked men dressed – like them – in black t-shirts. “We thought they were triads and started fighting with them,” one protester told public broadcaster RTHK.

“Suddenly, 60 to 70 riot police officers rushed towards us, pushed many people to the ground and arrested them, but I escaped.”

The suspected plainclothes officers ignored questions from reporters when confronted about their identity.

Teargas floods streets as Hong Kong protests continue Show all 31 1 /31 Teargas floods streets as Hong Kong protests continue Teargas floods streets as Hong Kong protests continue Riot police advance to disperse anti-government protesters in Hong Kong on 11 August EPA Teargas floods streets as Hong Kong protests continue Riot police fire teargas towards pro-democracy protesters during a stand-off in the Wan Chai district in Hong Kong. Thousands of pro-democracy protesters hit the streets of Hong Kong for a tenth weekend in-a-row Sunday, again defying police who fired volleys of tear gas at several locations AFP/Getty Teargas floods streets as Hong Kong protests continue Protesters react with teargas canisters fired by riot policemen AP Teargas floods streets as Hong Kong protests continue Pro-democracy protesters throw back teargas fired by the police AFP/Getty Teargas floods streets as Hong Kong protests continue A pro-democracy protester is held by police outside Tsim Sha Tsui Police station during a demonstration against the controversial extradition bill in Hong Kong on August 11, 2019 AFP/Getty Teargas floods streets as Hong Kong protests continue A Pro-Beijing supporter (R) shouts at a pro-Democracy protestor in Sham Shui Po area of Hong Kong AFP/Getty Teargas floods streets as Hong Kong protests continue Police arrest anti-government protesters EPA Teargas floods streets as Hong Kong protests continue Police officers fire teargas as anti-extradition bill protesters demonstrate in Sham Shui Po neighbourhood Reuters Teargas floods streets as Hong Kong protests continue Police disperse pro-democracy protesters outside Tsim Sha Tsui Police station AFP/Getty Teargas floods streets as Hong Kong protests continue An injured anti-extradition bill protester receives medical assistance Reuters Teargas floods streets as Hong Kong protests continue Anti-government protesters carry U.S. national flags and placards that appeal to President Trump to 'liberate Hong Kong' during a rally at Victoria Park in Hong Kong EPA Teargas floods streets as Hong Kong protests continue A family shares an MTR train car with anti-extradition bill protesters, who escaped from clashes with police in Kwai Fong Reuters Teargas floods streets as Hong Kong protests continue Pro-Beijing supporters (R) scuffle with an unidentified man in Sham Shui Po area of Hong Kong AFP/Getty Teargas floods streets as Hong Kong protests continue A protester wearing a military costume waves a US flag AP Teargas floods streets as Hong Kong protests continue Riot police arrest anti-government protesters outside the Tsim Sha Tsui police station EPA Teargas floods streets as Hong Kong protests continue Riot police separate members of the media from a group of pro-Beijing supporters after the supporters lunged towards the press at North Point AFP/Getty Teargas floods streets as Hong Kong protests continue A man helps put up a banner that reads 'Please don't come interfere with our freedoms, please go back!' AFP/Getty Teargas floods streets as Hong Kong protests continue An injured anti-extradition bill protester receives medical assistance Reuters Teargas floods streets as Hong Kong protests continue A protester throws a teargas canister fired by riot policemen AP Teargas floods streets as Hong Kong protests continue A demonstrator tries to extinguish a teargas canister Reuters Teargas floods streets as Hong Kong protests continue Protesters with protection gears face with riot policemen on a street AP Teargas floods streets as Hong Kong protests continue An injured anti-government protester is being treated by medics outside the Tsim Sha Tsui police station EPA Teargas floods streets as Hong Kong protests continue Anti-extradition bill protesters try to extinguish teargas canisters Reuters Teargas floods streets as Hong Kong protests continue Riot police arrest anti-government protesters outside the Tsim Sha Tsui police station in Hong Kong EPA Teargas floods streets as Hong Kong protests continue Hong Kong police stand guard as pro-China and anti-government protesters clash at North Point EPA Teargas floods streets as Hong Kong protests continue Policemen charge towards protesters outside the Tsim Sha Tsui police station in Hong Kong AP Teargas floods streets as Hong Kong protests continue Protesters jump over MTR gates as they move to another destination AP Teargas floods streets as Hong Kong protests continue A woman reacts from teargas inside a MTR station AP Teargas floods streets as Hong Kong protests continue Riot police officers Reuters Teargas floods streets as Hong Kong protests continue A man scuffles with riot policemen after he refuse to let policemen search his bag, in North Point neighborhood AP Teargas floods streets as Hong Kong protests continue Pro-democracy protesters react from tear gas fired by the police outside Tsim Sha Tsui Police station AFP/Getty

Other videos posted online showed police firing tear gas into a train station and firing what appeared to be pepper-spray at protesters at point-blank range.

Tear gas was also deployed in central Hong Kong on both sides of Victoria Harbour, in the Tsim Sha Tsui area on the Kowloon side and in Wan Chai on Hong Kong Island.

A protest outside Tsim Sha Tsui police station also left one woman protester with a bleeding eye – reportedly when she was hit with a bean-bag round in the face – while an officer suffered burn injuries to his leg after being hit by a petrol bomb.

Police defended their use of “decoy officers” during the demonstrations at a press conference on Monday evening. “Our decoy officers do not take part in any unlawful activities,” said deputy commissioner Tang Ping-keung, without going into further details.

The force said it was still reviewing the use of pepper spray pellets and was investigating whether its officers were responsible for the injury to a young female protester who was pictured with a bleeding eye.

It claimed that the use of tear gas was necessary to disperse violent demonstrators but admitted that some of the canisters had passed their expiry date.

Police also claimed that protesters were “posing a serious safety threat” by aiming laser pointers at officers outside another station in Kwai Chung.

On Monday, as the demonstrations stretched to a tenth week, thousands of people gathered at Hong Kong International Airport, leading to the cancellation of all remaining flights from around 4pm.

It followed reports that the territory’s flagship airline Cathay Pacific had sacked two members of staff and suspended a pilot over their alleged involvement in the protests.

Earlier in the day the Hong Kong Police Force had invited politicians and journalists to a demonstration of their crowd-control tactics using armoured cars equipped with water cannon.

Human rights group Amnesty International urged police to use “extreme caution” when deploying the equipment.

“Water cannons are not a toy for the Hong Kong police to deploy as a sign of strength,” said Hong Kong director Man-kei Tam.

“These are powerful weapons that are inherently indiscriminate and have the potential of causing serious injury and even death. In Hong Kong’s crowded streets, their deployment could be a recipe for disaster.”

Allegations of excessive force by police have prompted a petition to the US government calling for the Hong Kong police to be recognised as a “terrorist organisation”. The White House is required to respond as it has already attracted more than 120,000 signatures.

Hong Kong, a former British colony, was returned to China in 1997 after the Communist Party-led government agreed that the city’s democratic freedoms would remain unchanged for 50 years.

However mass protests erupted in June over an extradition bill that would have allowed Hong Kong residents to be sent to the mainland to stand trial.

The bill was later withdrawn but activists are now demanding the resignation of Hong Kong leader Carrie Lam, democratic elections for her successor, the release of those arrested in earlier protests and an investigation into police use of force.

Banners at the rally in Victoria Park read “Give Hong Kong back to us” and “Withdraw the evil law.”