Plainclothes Police Officer violently beat up restrained citizen

Several citizens in black shirts were arrested by undercover police officers near the Tsuen Fung Centre Shopping Arcade in Tusen Wan.
During the arrest, surrounding pedestrians and onlookers criticized the police officers of using unnecessary force and using police batons to repeatedly hit the subdued arrestee.
One of the female onlookers questioned the legitimacy of such arrest and asked the undercover police officer to show his identification card. The undercover police officer was able to present his identification card after several verbal exchanges with the female onlooker.
One of the arrestee had to repeatedly shout his name during the arrest to clearly identify himself.