Hong Kong police have admitted to deploy undercover officers among pro-democracy demonstrators to help catch 'violent protesters'.

Some policemen disguised themselves as 'different characters' in some 'decoy operations', a spokesperson said yesterday without going into details.

The statement came after footage of one bleeding demonstrator being pinned down to the ground had gone viral.

A trending clip, filmed by Hong Kong Free Press, shows one suspected undercover Hong Kong police officer apparently helping his uniformed colleague to pin a protester to the ground

Hong Kong police yesterday admitted at a press conference that they had used 'decoy officers' during demonstrations to help catch 'violent protesters'. The police declined to give details

The young man, who said his name was Chow Ka-lok and asked for a lawyer, was shown with a bleeding head wound and said he had a broken tooth. The clip has sparked public outrage

In the clip, the man was arrested by one uniformed police officer who was aided by two men suspected to be secret policemen dressed as activists.

One of the suspected officers was wearing a yellow hardhat and a black T-shirt, which is the typical outfit of anti-extradition-bill protesters. The other was wearing a white hardhat and a black T-shirt.

The three-and-a-half-minute-long clip was filmed by website Hong Kong Free Press. The news outlet said the incident occurred in Causeway Bay on Sunday night.

The young man, who said his name was Chow Ka-lok and asked for a lawyer, was shown with a bleeding head wound and said he had a broken tooth.

Yellow hardhats and black tops have become the iconic outfit for Hong Kong pro-democracy protesters who began their movement in June to oppose to a now-suspended extradition bill

Hong Kong has been rocked by protests for the past two months against a now-suspended bill that would allow people to be extradited from the city to stand trial in courts in mainland China

The city's leader Carrie Lam today reiterated her support for the police and said they had had to make on-the-spot decisions under difficult circumstances, using 'the lowest level of force'

The Hong Kong police attempted to fend off criticism yesterday after being accused of using excessive violence during clashes on Sunday night.

The police's actions sparked more protests in the city's bustling airport.

Some 5,000 demonstrators reportedly rallied at the terminal building yesterday to denounce police violence, forcing the airport authority to cancel all inbound and outbound flights from early afternoon. The airport has re-opened today.

Other graphic images from Sunday's clashes showed a woman who was shot in the eye. Local reports suggested that the female protester was hit by a bean round in the face during a clash in Tsim Sha Tsui and faces losing her right eyeball as a result.

Apple Daily also released a video showing police shooting projectiles to a group of protesters from within a metre inside a subway station, the Hong Kong news outlet said.

The incident was said to take place at around 11pm on Sunday in the Tai Koo Station, a stop on the Hong Kong Mass Transit Railway (MTR) system. The clip also shows anti-riot police chasing protesters on escalators and brutally beating them with batons.

Shocking footage from local news outlet Apple Daily shows police firing projectiles to a group of protesters from within a metre inside the Taikoo MTR station on Sunday, the report said

The clip also shows police chasing protesters on escalators and beating them with batons

Multiple people were reportedly detained and injured during the clash, Apple Daily said

A combination picture shows protesters wearing an eyepatch in reference to a demonstrator that was injured at previous day's clashes with police during a protest in the city centre

In yesterday's press conference, Deputy Commissioner Tang Ping-keung acknowledged that police use decoy officers in some operations but would not go into details.

'Our decoy officers do not take part in any unlawful activities,' Tang insisted.

Police confirmed that officers fired one shot of tear gas into a train station Sunday, saying it was necessary to disperse violent protesters. Addressing criticism of riot police firing pepper spray pellets at close range, officials said the weapon was not lethal but they would review the incident.

They said they were still gathering evidence about whether a young female protester who was pictured with a bleeding eye was hit by police.

Tang also said that an order had been issued for officers to stop using 'expired' tear gas against demonstrators. The Deputy Commissioner confirmed that such ammunition had been previously used, adding it posed no additional health risk compared to normal tear gas.

Hundreds of Hong Kong protesters have started four days of rallies at the city's busy airport since Friday. The city's streets witnessed another weekend of violent clashes between activists and police. One female demonstrator faces losing her eye after allegedly being hit by a bean bag round by the police in Tsim Sha Tsui on Sunday. The incident has sparked more anger

A protester covers her one eye with a gauze during the mass demonstration at Hong Kong International Airport today after news of a woman shot in the eye during a protest shocked the financial hub. Pro-democracy leaders were calling for as many as one million people to head to the Hong Kong airport today after 40 people were injured across the city over the weekend

A protester lays on the floor as she occupies the arrival hall of the Hong Kong International Airport during the mass demonstration today. Police have also reported injuries among their ranks during weekend's clashes, including eye irritation from laser pointers and burns from a petrol bomb. The city's anti-extradition-bill protests have lasted 10 weeks so far

Hong Kong's Airport cancelled all flights yesterday afternoon due to the demonstration

Hong Kong's leader Carrie Lam today defended police actions after protesters yesterday called for investigation into alleged police brutality during a 5,000-strong rally at the airport.

Ms Lam told reporters that dialogue would only begin when the violence stopped.

She reiterated her support for the police and said they had had to make on-the-spot decisions under difficult circumstances, using 'the lowest level of force'.

'After the violence has been stopped, and the chaotic situation that we are seeing could subside,' she said, 'I as the chief executive will be responsible to rebuild Hong Kong's economy … to help Hong Kong to move on.'

She did not elaborate on what steps her government will take toward reconciliation.

Hong Kong has been rocked by protests over the past months against a now-suspended bill that would allow people to be extradited from the city to stand trial in Communist Party-controlled courts in mainland China.

Anti-extradition bill protesters react from tear gas as riot police try to disperse them during a protest at Sham Shui Po in Hong Kong in the 10th week of violent showdowns

Police arrest anti-government protesters during a rally in Sham Shui Po in yet another day of demonstrations that has plunged Hong Kong into its most serious crisis in decades

Protesters with umbrellas and homemade shields as they face with riot policemen on a street in Hong Kong as police fired tear gas late Sunday afternoon to try to disperse a demonstration

The mass display of opposition to the bill has morphed into a wider pro-democracy movement that has thrown down the most significant challenge to Beijing's authority since the former British colony returned to Chinese rule in 1997.

No new violence has been reported since Sunday, although the city is on edge after more than two months of near-daily and increasingly bloody confrontations between protesters and police.

Beijing yesterday slammed violent Hong Kong protests as 'terrorism' and said the city was at a critical juncture.

Pro-Democracy protesters throw back tear gas fired by the police during a demonstration against the controversial extradition bill in Sham Shui Po district

An anti-extradition bill protester throws a tear gas canister during a demonstration in Wan Chai neighbourhood in Hong Kong

Police officers fire tear gas as anti-extradition bill protesters demonstrate in Sham Shui Po neighbourhood in Hong Kong

'Hong Kong's radical demonstrators have repeatedly used extremely dangerous tools to attack police officers, which already constitutes a serious violent crime, and also shows the first signs of terrorism emerging,' said Yang Guang, a spokesperson at China's Hong Kong and Macau affairs office.

State newspaper People's Daily also released footage of military vehicles gathering near the border of Hong Kong and warned that armed police could be mobilised to crack down on 'riots and terrorist attacks'.

Chinese state media today ramped up the rhetoric against Hong Kong's pro-democracy campaigners, describing them as 'mobsters', warning they must never be appeased and raising the spectre of mainland security forces intervening to quash them.

Hong Kong's last British governor Chris Patten today cautioned that if China intervened in Hong Kong, it would be a catastrophe and that Chinese President Xi Jinping should see the wisdom of trying to bring people together.

Patten said it was counter-productive of the Chinese to warn of 'other methods' if the protests did not stop.