The California Highway Patrol was last night facing a potential federal investigation into the circumstances that led to the “excessive and brutal” repeated punching of a prostrate and apparently unarmed black woman by a Patrol officer.

Police in California are investigating the incident which was recorded on a mobile phone. Civil rights groups and community activists demanded a federal probe. In the video, posted on YouTube and viewed by tens of thousands of people at the time of writing, the officer, reportedly since placed on “administrative leave”, was seen delivering several blows to the woman’s face on the side of a busy road.

The footage appeared to show the Highway Patrol officer stopping a woman on a busy Los Angeles motorway, before pinning her to the ground and punching her in the face. The woman seemed to be walking away from the officer before the confrontation.

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Speaking to the television station ABC7, the Californian Highway Patrol said that the officer had ordered the woman to stop walking, out of fears for her safety. “A physical altercation ensued as the pedestrian continued to resist arrest, at which point a plain clothes, off duty officer assisted in applying the handcuffs.”

David Diaz, who filmed the incident from his car on Tuesday evening, told local television station CBSLA that the officer’s behaviour was “excessive and brutal”. He said: “If you look at the video, there are 15 hits. To the head, and not just simple jabs. These are blows to the head, really serious blows. I find it hard to believe there was no other remedy in this situation.”

Policing expert and Associate Professor in the School of Criminal Justice at Michigan State University, William Terrill, told The Independent on Sunday: “If the officer thought she had a gun, or if she had made a comment about having a gun or a bomb, then him being alone in that situation on the highway might justify that level of force. He had the right to restrain her but he certainly didn’t have the right to punch her in the face.”

He added: “It seems like there was a safety threat there but by law, the level of force has to be proportionate to the level of citizen resistance so in this case the resistance appears to be that she didn’t follow his verbal command to stop her. Unless she aggressively resisted, him attacking her in that way is unlawful.”

The video, which has since gone viral, is not the first of its kind to seemingly depict US police authorities restraining women with blows to the face. In July 2012, Fox News obtained mobile phone footage in which two police officers in San Antonio appeared to be beating a pregnant woman.

Professor Terrill, who is also a former military police officer, said: “The more accountability from capturing these things on audio or video, the better. I think it shows that the type of force which is used needs to be put into proper scope and if that’s captured on video, that puts officers on alert: they have to abide by the law.”

The incident is the latest in which police officers in Los Angeles have faced accusations of excessive force.

Earlier this year David Cunningham, a black American judge, filed a $10 million claim alleging racial profiling and excessive force after he was handcuffed and put into a police car when he was stopped for not wearing a seat belt.