Video footage showing officers restraining a man as he repeatedly shouts “I can’t breathe” has been referred to the police watchdog.
A man can be seen lying on the ground as he is restrained by three officers near a police car in a recording captured in Brighton on Tuesday.
In the video, the man can be heard saying: “Why have you got your arm on my neck? Bro, I can’t breathe.”
An officer replies, saying the arm is on the man’s collarbone, not his throat.
A woman who saw the man being “tackled” to the ground described the incident as “intense”.
The witness, who did not see what happened beforehand, told PA news agency: “I have seen people be arrested before, I have never seen anything like that.
“It was really, really shocking. I do not understand why there were three policemen sat on him.”
Sussex Police said the 28-year-old man had refused officers entry to a property in Montpelier Road while they were searching for a missing vulnerable teenager.
The force said the man became aggressive towards officers after being arrested and was subsequently placed in handcuffs on the ground.
The 17-year-old girl was found at the property and taken home by officers, it added in a statement.
“Once under arrest, the man became aggressive towards officers and was handcuffed and placed on the ground before being transported to custody,” the force said.
“We train our officers to protect themselves and others using reasonable force and are reviewing this footage, together with body-worn video captured by the officers of the entire interaction, to identify if any further investigation or learning is required.”
Brighton and Hove policing commander Chief Superintendent Nick May said the force understood some may find the video shocking but that it showed a “small part” of a much longer interaction and that it was “important to understand the wider context”.
The force has voluntarily referred itself to the Independent Office for Police Complaints (IOPC) for investigation.
A spokesman for the IOPC said: “We are aware of this matter and are making further inquiries with Sussex Police.
“We will then be in a position to make an informed decision on the level of IOPC involvement.”
Additional reporting by PA