A police officer in the UK has been filmed punching a man in the face as he was being detained under the Mental Health Act.
The mobile phone video shows the handcuffed man being held down by three officers, one of whom is seen drawing back a clenched fist.
The officer is then seen hitting the man in the face, before he is turned over and led away following the incident in Yeovil, Somerset, on Wednesday.
In response to the footage being posted online, Avon and Somerset Police released its own 24-second clip of the arrest, which was filmed on an officer's body-mounted camera.
The police footage includes audio and shows a different angle as the man was repeatedly ordered to let go of an officer's arm.
A male officer tells him: "Let go of my arm now. I will ask you one more time to let go of my arm. Let go of my arm. Five..."
An onlooker interrupts and says: "Go on mate punch him."
The male officer continues: "Four... three..."
A female officer adds: "Let go of his arm."
Another voice can be heard saying in the background: "Punch him in the face."
Moments later an officer can be heard striking the man, who releases his grip a few seconds later.
A witness then says: "Oh, that's a bit naughty. That is definitely on video. That's on Facebook, mate."
The original video was put online without audio, but offers a clearer view of the punch.
Luke Harris, who posted the video on his Facebook page, wrote: "This guy was detained under the Mental Health Act.
"Whilst three officers had him in cuffs, he was punched without being able to defend himself. I think this is wrong."
Avon and Somerset Police said its professional standards department was investigating the incident.
"We were called shortly before 5pm from the ambulance service regarding a man acting aggressively towards their staff," a force spokesman said.
"It appeared the man was suffering from a mental health episode, so officers took the decision to detain him under the Mental Health Act.
"This decision is always taken to safeguard the individual from harming themselves and others.
"While we always act in the best interests of the person in crisis, there is sometimes a need for restrain to protect the person, our officers and members of the public.
"This man was taken to a mental health facility where he can receive the treatment he needs.
"The public rightly expects police officers to use force reasonably so we welcome their scrutiny.
"Our professional standards department will now make their own enquiries into this matter."