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.

'A whole civilisation will die tonight,' Trump says as deadline looms
China and Russia veto UN resolution on protecting Hormuz shipping
US hits military targets on Iran's Kharg Island, Vance says no change to strategy
One attacker dead, two injured in gunfight outside Israeli consulate in Istanbul
