Men in white t-shirts, some armed with poles, stormed a subway station and attacked passengers. Witnesses said the masked men were attacking people that had been at the anti-government march.
Hong Kong police on Monday defended their actions and the lack of arrests after opposition lawmakers said police had failed to protect a train full of anti-government protesters from a triad gangster attack on Sunday.
The attack came in a night of violence that opened new fronts in Hong Kong's widening political crisis over an extradition bill to send people to China for trial.
Some politicians and activists have long linked the allegedly Chinese network of triad criminal gangs to political intimidation and violence in recent years.
On Sunday night, men in white t-shirts, some armed with poles, flooded into the rural Yuen Long station and stormed a train, attacking passengers, according to footage taken by commuters and Democratic Party lawmaker Lam Cheuk-ting.
Protesters target of attack?
Eye-witnesses said they appeared to target black-shirted passengers who had been at an anti-government march and the attack came after several thousand activists surrounded China's representative office in the city, later clashing with police.
Hong Kong media have released video footage showing masked assailants attacking commuters in a subway station. Among those attacked were protesters clad in their trademark black clothing and yellow hard hats.
Lam, who was injured in the attack, said he was angry about a slow police response after he alerted them to the trouble, government-funded broadcaster RTHK reported.
Lam said it took police more than an hour to arrive after he alerted them and they had failed to protect the public, allowing the triads to run rampant.
The party is now investigating.
"Is Hong Kong now allowing triads to do what they want, beating up people on the street with weapons?," he asked reporters.
One government worker, aged 22, described chaotic scenes as people ran in disarray off the train, despite word that gangsters were attacking people on the street as well.
"People started to run in all directions, just avoiding the gangsters," he said.
Police said early on Monday they had not made any arrests at the station or during a follow-up search of a nearby village but were still investigating.
Yau Nai-keung, Yuen Long assistant district police commander, told reporters that an initial police patrol had to wait for more reinforcements given a situation involving more than 100 people.
Groups of men in white were seen by eye-witnesses with poles and bamboo staves at a nearby village but Yau said police saw no weapons when they arrived.
"We can't say you have a problem because you are dressed in white and we have to arrest you. We will treat them fairly no matter which camp they are in," Yau said.
Hong Kong's anti-triad police units in 2014 investigated the role of triad gangs attacking protesters on the streets of Mong Kok during the 'Occupy' pro-democracy demonstrations that shut down parts of the city for 79 days.
Hong Kong has been rocked by a series of sometimes violent protests for more than two months in its most serious crisis since Britain handed the Asian financial hub back to Chinese rule in 1997.
Protesters are demanding the full withdrawal of a bill to allow people to be extradited to mainland China for trial, where the courts are controlled by the Communist Party, fearing it would undermine Hong Kong's judicial independence.
They are also demanding direct elections and independent enquiries into the use of police force against protesters.
On Sunday, police fired rubber bullets and tear gas to disperse activists on the edge of Hong Kong's glittering financial district after they had fled China's Liaison Office, its main representative branch in the city.
The Chinese government, including office director Wang Zhimin, condemned the acts, which saw signs and a state symbol daubed with graffiti and eggs thrown against walls and windows.
Wang said that a damaged symbol had been replaced overnight.
The unrest in Hong Kong marks the greatest popular challenge to Chinese leader Xi Jinping since he came to power in 2012.
The Hospital Authority said 45 people were injured in the Yuen Long attack, with one in a critical condition. Some 13 people were injured after the clashes on Hong Kong island, one seriously, the authority said.
Some police had been injured in the clashes after protesters hurled bricks, smoke grenades and petrol bombs, said a police statement.