Dozens of masked men armed with batons stormed a train station in the Hong Kong district of Yuen Long on Sunday.
The men, all in white T-shirts, violently attacked 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.