Masked men attacked passengers after storming a train station in Hong Kong's Yuen Long district on Sunday.
Police say dozens of people are injured with one person in critical condition.
It comes after the latest pro-democracy rally in the centre of Hong Kong.
Demonstrations were also protesting against a controversial bill that allows extraditions to mainland China.
It's still unclear who the mob was or why the attacked the passengers.

Trump warns Britain on China ties as Starmer hails progress in Beijing
Israel releases 15 Palestinian bodies as truce deal shifts to next phase
WHO sees low risk of Nipah virus spreading beyond India
Trump, Democrats say deal reached to avert shutdown
Vietnam police seize tonnes of fake coffee products made from soybeans