Hong Kong police are facing accusations of failing to protect pro-democracy activists who were attacked by unidentified assailants on Sunday.
Opposition lawmakers say police inaction allowed criminals to beat up peaceful demonstrators after a rally.
Dozens were injured when they were set upon by masked men wielding sticks.
Police say their forces were stretched during another day of unrest. Hong Kong has been rocked by weeks of protests sparked by an extradition bill.
Late on Sunday video footage showed groups of men – dressed in white shirts and suspected to be triad gangsters – beating passengers with rods at a train station in the Yuen Long area.
There is speculation that the attackers have links with criminal gangs, known as triads. Forty-five people were injured, with one person in critical condition.
Hong Kong’s Chief Executive Carrie Lam described the attacks as “shocking”. She also condemned protesters for defacing China’s main representative office in the city earlier in the day.