Seattle police are to review training procedures after video of an officer punching a teenage girl circulated widely on US television and online.
The officer struck the 17-year-old after she tried to intervene while he was scuffling with another teenage girl he had seen jaywalking, officials said.
The Seattle police union defended the officer, saying he was working alone amid a crowd and feared for his safety.
The girls, who were arrested, were not harmed in the incident, police said.
In the video, shot on Monday, the officer is shown struggling with a girl in black who police said had tried to leave the scene when he confronted her about jaywalking.
A girl in a pink top intervenes, apparently trying to free the other girl. The footage then shows the police officer punching her as onlookers watch.
The officer, Ian Walsh, has been transferred temporarily to the department's training unit, Seattle police said.
Police officials said the review of training procedures was not meant as a criticism of the officer's actions but would examine whether training could be improved.
Seattle police officers' guild president Richard O'Neill said officers were wary of crowds that could turn violent.
"This officer, if you see on the video, was surrounded by an ever-growing group," he said. "This could have been a tragedy."
A Seattle police spokesman said that use of force was within an individual officer's discretion.