Scotland Yard is investigating allegations that one of its officers repeatedly punched a detainee, after footage of the apparent incident was posted on the internet.
A three-minute video on the Daily Mail website appears to show a male officer striking a topless man at least six times and telling the suspect to "shush".
The footage, said to have taken place over the summer in the Ruislip area of west London, shows four officers in short sleeve shirts appearing to detain the suspect, who repeatedly howls in pain and begs for mercy.
The suspect, who is apparently under arrest for assault, can be heard shouting: "No, please. Oh my goodness, no, no, no, no, please."
The suspect is then told: "If you spit, I'll f***ing hurt you."
A female officer tells the suspect, who has his head covered, to calm down, before he is put in the back of the police van.
A Metropolitan Police spokesman said: "We are looking into it (the video)."