The footage, which has been shared with the Guardian, shows Kayvan Sabehgi standing in front of a police line on the night of Occupy Oakland's general strike on 2 November, when he is set upon by an officer.
He does not appear to be posing any threat, nor does he attempt to resist, yet he is hit numerous times by an officer clad in riot gear who appears determined to beat him to the ground.
Sabehgi, 32, an Oakland resident and former marine who served in Iraq and Afghanistan, has since undergone surgery on his spleen. He says it took hours for him to be taken to hospital, despite complaining of severe pain. Police have told the Guardian they are investigating the incident.
The footage was recorded by artist and photographer Neil Rivas, who said Sabehgi was "completely peaceful" before he was beaten. "It was uncalled for," said Rivas. "There were no curse words. He was telling them he was a war vet, a resident of Oakland, a business owner."
Sabehgi has previously said he was talking to officers in a non-violent manner prior to his arrest, which the footage appears to confirm.
The 32-year-old can be seen standing in front of a line of police officers, all of whom are in riot gear. The officers walk forward, chanting and thrusting their batons, and Sabehgi starts to walk backwards.
Although the video is dark, an officer can clearly be seen beginning to hit Sabehgi around the legs with a baton, then starting to strike him higher up.
Sabehgi then appears to be bundled to the ground. He was later arrested.
Rivas said the footage was shot around midnight on 3 November, as police approached Occupy Oakland following the 2 November general strike.
Police deployed teargas and non-lethal projectiles that night, after some protesters entered a disused building north of Frank H Ogawa Plaza, but Rivas said there did not appear to be an immediate threat to police at the time of the video.
"It was pretty much just Kayvan and myself right there at that moment when he got beat," Rivas said.
"I couldn't help but start yelling out for them to stop. He was not fighting back; he was moving away from the officer. It did not feel good.
"I saw him being taken down to the ground and I tried to keep my camera focused on that as well, but they were pretty quick at setting up a barricade between myself and Kayvan at that point. I was shoved out of the way, and I had several guns pointed my way.
"I remember specifically one officer right in front of me having his gun pointed point blank at me."
Rivas said he realized the man in his video was Sabehgi after reading that a second Iraq war veteran had been injured, and seeing television footage.
Oakland television station KTVU TV-2 has previously shown footage of Sabehgi in handcuffs just after he was arrested, but Rivas believes this is the only video of him being beaten.
Sabehgi has previously told the Guardian he was walking away from the main area of police clashes – at 16th Street and Telegraph, just north of the Occupy base at Frank H Ogawa Plaza – when he was beaten and arrested.
Several police agencies were involved in the operation on 2 and 3 November, but Rivas said the officers who appear in front of Sabehgi at the beginning of the video were from Oakland police department.
A spokeswoman for Oakland police said: "The Oakland Police Department is currently investigating the incident."