South Wales Police say an incident when a man appeared to be tasered and hit by officers has been referred to their professional standards department.
The scene, recorded on a mobile phone video in Swansea last Wednesday, has been released by the Sun newspaper.
The 35-second video appears to show two officers struggling on the ground with the unnamed man, who has been charged with criminal damage.
The 29-year-old was arrested at about 2315 BST on Wind Street.
The video appears to show one officer using a Taser stun gun on the man's lower body, before a second policeman strikes him four times on or near his head.
One of the officers then tells a crowd, who can be heard out of shot, booing: "He's resisting - go away."
The force said their professional standards department would look at the footage.