A police officer armed with a Taser kicked and stamped on a man’s head as he was lying face down on the ground at Manchester Airport.
The incident was filmed and has been widely circulated on social media.
A second man was also struck by the officer.
The video is being “assessed” by professional standards, Greater Manchester Police has said.
Eyewitness Amar told Sky News that he had heard shouting as he was turning the corner in the area. He heard: “you’re under arrest, you’re wanted.”
He says he then saw one man being “pinned against the wall”, before “pushing and shoving.”
Amar says he believes it was then that the man’s brother came “running over” before being pushed to the floor and “kicked” by police.
Greater Manchester Police added firearms officers had been responding to reports of an altercation between members of the public.
The force said a female officer involved suffered a broken nose and that all three responding officers were taken to hospital.
The force said “there was a clear risk during this assault of their firearms being taken from them,” and said they “acknowledge the concerns of the conduct within the video”.
Four men were arrested for affray and assault on emergency service workers, police said.
Manchester Airport said: “We are aware of an incident involving Greater Manchester Police officers that took place yesterday and, as a result, the force is investigating the matter.
“We will support the investigation in any way possible.”
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