Hong Kong has been left in shock after a night of violence on Sunday, which saw dozens of masked men storm a train station.

The men – dressed in white shirts and suspected to be triad gangsters – assaulted pro-democracy protesters and passers-by in the Yuen Long area.

This is the first time this kind of violence has been seen in the ongoing anti-extradition demonstrations. Several lawmakers questioned why police were slow to arrive at the scene.

Footage posted on social media showed dozens of men attacking people with wooden rods and metal sticks inside the station. Forty-five people were injured, with one person in critical condition.

Hong Kong’s Chief Executive Carrie Lam told reporters the gang attacks were “shocking”. She also condemned protesters for defacing China’s main representative office in the city earlier on Sunday.

Pro-democracy protesters were set upon as they travelled back from a rally in the centre of Hong Kong, where riot police had fired tear gas and rubber bullets at protesters. The masked men stormed Yuen Long MTR station at about 22:30 local time (14:30 GMT).

Local media said they were targeting people dressed in black – the colour most protesters were wearing.

In a statement, the government said: “This is absolutely unacceptable to Hong Kong as a society that observes the rule of law. The SAR [Special Administrative Region] Government strongly condemns any violence and will seriously take enforcement actions.”

One journalist, Gwyneth Ho, was attacked while she was in the middle of live streaming for news website Stand News. She is currently in hospital.
The Hong Kong Journalist Association said some reporters on the scene had equipment seized.

One witness – Galileo Cheng, 34 – said he had suffered several blows to his back and arms when he stepped in to try to help a female journalist, who was under attack. He said he saw another woman with a baby being assaulted too. It is not known who organised the attack.

Opposition lawmaker Lam Cheuk-ting suggested the mob had ties to organised crime syndicates. “Is Hong Kong now allowing triads to do what they want, beating up people on the street with weapons?” he asked reporters.

Another pro-democracy lawmaker, Ray Chan, tweeted a complaint about the police response. “Hong Kong has one of the world’s highest cop to population ratio … Where were [they?]”

Video footage also surfaced of pro-Beijing lawmaker Junius Ho, shaking hands with men in white and giving them the thumbs-up signal. On Facebook, he denied a connection with the men and said he was just responding to their greetings when they recognised him on the street after he left a restaurant. His office was later ransacked. Police on Monday said they had not made any arrests but were still carrying out investigations.

Some of the assailants did not seem too concerned about masking their identity, posting selfies online, before during and after the attack.