A mob 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.
At least 36 people were injured in the violence, local media report.
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 or what the motives for the attack were.