Video shot by a motorist showed a CHP officer throwing a woman to the ground, straddle her body and repeatedly punching her. (Credit: David Diaz)

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A motorist used his cellphone to capture what appeared to be the violent arrest of a woman on the 10 Freeway near La Brea Avenue on Tuesday.

The video, shot by David Diaz, showed the woman walking along the freeway when she was approached by an unidentified California Highway Patrol officer.

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It was not clear if words were exchanged between the two, but at one point the officer walked up behind the woman and grabbed her, the video showed.

A struggle ensued, during which the officer appeared to knock the woman to the ground, straddled her body and began repeatedly punching her.

Diaz, who posted the video on YouTube, was disturbed by what he witnessed.

"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.

Another man soon ran up to the officer and appeared to be helping him restrain the woman, the video showed.

The video did not capture the circumstances that led up to the incident.

Still, Diaz believed it was clearly a case of excessive force.

"There is no way you can justify that," he said.

Late Thursday, CHP spokeswoman Sgt. Denise Joslin issued the following statement:

"The California Highway Patrol (CHP) is aware of the video and we are looking into the incident. 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." Joslin 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," she said.