Media playback is unsupported on your device Media caption The unidentified officer involved in the incident is reported to be on administrative leave while it is being investigated

The California Highway Patrol says that it is investigating footage posted on YouTube that shows a policeman repeatedly punching the face and head of a prostrate woman.

Video shows him straddling the woman and hitting her at least 11 times.

Police say that the woman needed to be restrained and was endangering herself and motorists by walking on a busy road in the west of Los Angeles.

The incident was recorded by passing motorist David Diaz.

"[The policeman] just pounded her," he told CBS Los Angeles.

"If you look at the video, there are 15 hits. To the head, and not just simple jabs. These are blows to the head. Blows. Really serious blows. And this is ridiculous to me."

"I find it hard to believe there [was] no other remedy in this situation."

The officer involved in the incident is reported by the AP news agency to be on administrative leave while it is being investigated. He has not been identified.

The California Highway Patrol (CHP) says that it is looking into the details of the incident, which has been condemned by civil rights leaders.

"As a matter of policy, every time there is a use of force by our officers, there is a review conducted to determine whether the use of force was appropriate," a CHP statement said.

"That will be done in this case. However, since there is an ongoing investigation, it would be premature to comment on this specific video segment without reviewing the entire incident."

When the video starts, the officer is seen trying to detain the barefooted woman, who walks a few steps away from him.

But the officer is seen forcing her to the ground, briefly struggling with her before repeatedly pummelling her. A few moments later, a plainclothes officer enters the picture and helps his colleague put the woman in handcuffs.

A CHP spokesperson was quoted by CBS Los Angeles on Friday as saying that the agency was trying to bring Tuesday's incident "to a just conclusion".