Acting Deputy Police Chief Nick Metz said Tuesday that the department's civilian-led Office of Professional Accountability is investigating the actions of Officer Ian P. Walsh, 39.
He was trying to cite several people for jaywalking just before Monday's incident, which was captured on cell phone video.
It shows Walsh trying to arrest 19-year old Marilyn Levias. The two were already struggling when Levias' friend, a 17-year-old, pushed the officer, and he responded by punching the friend in the face.
"Are you serious? Are you serious?" bystanders are heard saying.
Metz says the two teens in the video bear much of the responsibility for not cooperating and resisting arrest.
The Seattle Police Officers' Guild is defending Metz, saying, if he made any mistake, it was in waiting to use force. "He did nothing wrong," insists the Guild's Rich O'Neill. "I think he was trying to defuse the situation and calm people down."
O'Neill also says the punch that bruised Levias' face is what officers are trained to do. "If they had an abrasion or scrape or whatever, oh, well: They should have thought of that when they put their hands on the officer," O'Neill contends.
But leaders of several community groups, while not excusing the suspects, say Walsh clearly overreacted.
"The use of violence in the form of a full-blown fist to the face was wrong," Seattle Urban League President and CEO James Kelly told reporters. He says the punch brought to mind a video taken April 17 of two Seattle officers seen kicking a Hispanic suspect.
The 17-year old was in court Tuesday, charged with assaulting an officer and released.
Levias was arrested on charges of obstructing a police officer.
CBS affiliate KIRO-TV found court records showing Levias has been arrested for assaulting a police officer before. According to the documents, Levias kicked a King County Sheriff's deputy in the stomach while she resisted arrest at the Ruth Dykeman Center in Burien.
Walsh has been temporarily re-assigned.
To see the incident, click on the video below of a report by Tarar Mergener, introduced by "Early Show" newsreader Chris Wragge: