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 vows no more attacks by Israel on Iran gas field
Israel reopens key Gaza crossing amid new truce push
Oman condemns attacks on UAE, Qatar, Saudi Arabia
Saudi Arabia trust with Iran hard to 're-establish', foreign minister says
Kuwait Petroleum Corporation contains fire after drone hits refinery
