The incident happened at the Prince Edward Subway station in Kowloon, Hong Kong on the 31st of August 2019. Prince Edward station is far away from the protest venue, which is in Hong Kong island. There have been reports that the police arrived the scene after the train company called the police that someone was attacking the citizens. (news media later reported that it was a blue shirt elder attacking citizens with a hammer). The police then stormed in and use batons and pepper sprays to attack everybody with a surgical mask on or look remotely like a protestor.
Background: There was a massive protest in several districts of HK on the night of August 31th. Since the government in HK tried to push forward the extradition law that would have allowed extradition from Hong Kong to mainland China in early 2019, HK citizens have been protesting for months since June 2019.