A police officer in Los Angeles has been filmed repeatedly punching a woman in the face by a passing motorist who uploaded the footage to YouTube.
Warning: Video contains graphic images
The officer is seen grabbing the woman as she walked on a central reservation next to a busy dual carriageway.
He then pulls her to the ground and straddles her before repeatedly punching her in the face and head.
A man in civilian clothing, later confirmed as an off-duty police officer, then joins the police officer to assist in restraining the woman.
Chris O'Quinn, California Highway Patrol Assistant Chief, said that the woman had been endangering herself and people in traffic, and the officer was trying to restrain her.
"What we understand to have occurred at this point was that she started to walk down the on ramp and then turned around and came back up and crossed lanes of traffic, endangering herself and the motoring public," he said.
"It was heavy traffic at that time, but people aren't anticipating a citizen being out there on the freeway walking."
He added: “The tape only shows a small part of what transpired, there were events that led up to this. Until all that’s collected and put into perspective we aren’t going to be able to make a determination.”
David Diaz, who released the video under the title 'Police Brutality?' told KTLA-5 that he believed the force used by the officer was "unjustified".
He said: "He basically got on top of her … it’s basically a UFC ground-and-pound move … full force, punching her in the head … in the head,” Diaz said.
“There is no way you can justify that,” he added.
The unnamed officer is on administrative leave while the investigation takes place.
The force said that the woman was uninjured and is undergoing psychiatric evaluation.