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.

Iran confirms death of security chief Larijani
Washington targets Iran's fortified missile sites near Hormuz
Israeli strikes in central Beirut kill 10 people, Lebanese authorities say
Israel military says its tank fire hit UN Lebanon base, regrets incident
Trump says NATO's refusal to help on Iran is 'very foolish'
