Interim Baltimore Police Commissioner Gary Tuggle said Monday he was “disappointed” and “disturbed” by the viral video of one of his officers beating up a civilian Saturday on Monument Street.

It pointed out, he said, “another deficiency in our training that we can actually learn from.”

The officer, identified as Arthur Williams, graduated from the police academy last year.

A short video posted on social media Saturday shows Williams arguing with a man identified as DaShawn McGrier. Williams pushes McGrier into a wall then begins swinging at him, striking him in the head and face and taking him to the sidewalk.

Williams was suspended with pay after video surfaced and Tuggle accepted his resignation Monday. A second officer who was at the scene has been placed on administrative duties.

Mayor Catherine Pugh tweeted that she had seen the "disturbing video" of the incident and that she has "demanded answers and accountability."

"We are working day and night to bring about a new era of community-based, Constitutional policing," she tweeted, "and will not be deterred by this or any other instance that threatens our efforts to re-establish the trust of all citizens in the Baltimore Police Department."

Warren Brown, McGrier’s lawyer, charged in an afternoon news conference that Williams was mentally and emotionally unfit to be a police officer.

"When I saw this officer attack this guy it said to me, one, that this is personal, not policing," Brown said. "This is something personal."

Williams had a run-in with McGrier in June. In that case, he charged McGrier with assault, disorderly conduct, and resisting arrest. In a statement of probable cause William reported that McGrier threatened to kill him. That case is scheduled for trial next week.

Brown said he is seeking restitution from the city for McGrier’s injuries. McGrier was expected to be released from the hospital Monday, having been treated for a fractured jaw, a broken nose, and broken ribs.