Officer Bijan Darvish, who is white, said he punched 16-year-old Donovan Jackson twice before the teen was handcuffed.
"Fearing that Jackson would pull me into him and strike me with his other hand, I punched him two times in the face, using my right hand," Darvish wrote in the report, which was obtained by the Los Angeles Times on Friday.
Darvish is the partner of Officer Jeremy Morse, who has been placed on paid leave as federal, state and local agencies investigate the July 6 arrest of Jackson and his father at a gas station.
The tape shows Morse, who is white, slamming Jackson onto a patrol car and then punching him in the face.
Additional details of the arrest emerged as activists marched on City Hall, calling for changes in police procedure and demanding that the officers be fired and criminal charges be lodged against Morse.
"We want to know what kind of threat a 16-year-old eating a bag of potato chips poses to trained police," said Thandi Chimurenga, a member of the newly formed Donovan Jackson-Chavis Justice Committee.
In the police report, officers said Jackson repeatedly ignored their commands. Los Angeles County sheriff's Deputy Carlos Lopez wrote that he tried to get the teen to sit in a patrol car, but then Jackson lunged at him.
Darvish and Morse, who arrived at the scene to provide backup, said they saw Jackson hit the deputy. They then grabbed Jackson's arms and pulled him to the ground, the report said.
Officers said Jackson grabbed at Morse's shirt and scratched the officer once above the ear and then on his neck. Jackson then grabbed Darvish by the shirt and started pulling him down, the report said. Meanwhile, the report said Jackson continued to swing at Darvish.
"I yelled at Jackson to let go of my uniform; however, he refused," Darvish wrote.
It was then that Darvish punched Jackson, according to his account. Two other officers, Mariano Salcedo and Antoine Crook, helped handcuff Jackson. The rest of the arrest was caught on video.
An attorney for Jackson's family said he was skeptical of the police report, claiming all four Inglewood policemen "took turns" beating Jackson before the videotape began.
"What we see on the tape is the second beating," said attorney Joe Hopkins.
Morse's lawyer, John Barnett, said Morse had been "restrained" in his use of force, considering that Jackson had grabbed his testicles. He urged people not to rush to judgment about what they saw on the tape.
"Under the circumstances that have been described, he is entitled to punch the suspect when his testicles are being grabbed," Barnett said.
Meanwhile, Inglewood Mayor Roosevelt Dorn revealed that officials will re-examine two previous complaints made against Morse to "determine whether this kind of conduct has been perpetuated ... by the officer before."
Morse also was sued Friday by a woman who claims he and other law enforcement officers violated her civil rights. Patricia Surjue alleges Morse assaulted her in front of her two children when officers stormed into her home on Oct. 20.
Morse's lawyer did not immediately return telephone calls seeking comment about the federal suit.
The videotape was made by Mitchell Eugene Crooks, an unemployed Northern California man and a fugitive since 1999 when he was convicted of driving under the influence, hit and run and petty theft. He was flown to Auburn on Friday to serve a seven-month criminal sentence, Placer County sheriff's Capt. Rick Armstrong said.
At the Friday rally, protest leaders urged people to remain calm while the incident was investigated.
"While people are properly outraged in the community - as well they should be - it has not gotten out of control as of yet," John Mack, executive director of the Los Angeles Urban League, said at the rally Friday.
The hundreds of protesters at the demonstration demanded that criminal charges be filed against the policeman. Many chanted "No justice, no peace," and one carried a pink sign reading "To Protect & Serve, NOT Lie & Cover-Up."