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.
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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.
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