Dozens of masked men armed with batons stormed a train station in the Hong Kong district of Yuen Long on Sunday.
Footage posted on social media showed the men, all in white T-shirts, violently attacking people on platforms and inside train carriages.
Forty-five people were injured, with one person in a critical condition.
The mob attack followed the latest pro-democracy rally in the centre of Hong Kong, where riot police had fired tear gas and rubber bullets at protesters.
It is unclear who the mob were, but they attacked passengers and people making their way back from the protest.
One pro-democracy lawmaker questioned why police did not arrive sooner to break up the fight.
"Hong Kong has one of the world's highest cop to population ratio," said Ray Chan in a tweet. "Where were [they?]"
Another opposition lawmaker said police took more than an hour to arrive, and 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?" Lam Cheuk-ting asked reporters.
In a statement, the government said that in Yuen Long "some people congregated at the platforms of the MTR station and train compartments, attacking commuters".
BDST: 1035 HRS, JUL 22, 2019