Dozens of masked men attacked commuters and anti-government protesters inside the Hong Kong Mass Transit Railway (MTR) station of Yuen Long on Sunday, according to footage taken by commuters and posted on social media.
Footage shows men dressed in white T-shirts and armed with poles breaking into the Yuen Long station, where they reportedly stormed a train and attacked passengers that included people returning from an anti-government protest.
Forty-five people were reportedly injured, one of which in a critical condition.
Triad gangs have been suspected to be behind the violence, according to reports.
According to Democratic Party lawmaker Lam Cheuk-ting, who was injured in the attack, the police arrived more than an hour after he alerted them.
Police said early on Monday they had not arrested anyone at the station or during a subsequent search of a nearby village but that investigations were still ongoing.
The Democratic Party is now conducting an investigation on the attack.
The incident comes after thousands hit the streets of Hong Kong on Sunday to protest against the now-suspended extradition bill and demanded the resignation of Chief Executive Carrie Lam.