San Francisco's Police Chief Greg Suhr has stepped down hours after a police officer shot and killed a young black woman driving a suspected stolen car.
The resignation was announced by Mayor Ed Lee, who had asked him to quit.
Mr Suhr and city police had in recent months come under fierce criticism over fatal police shootings of several black suspects.
Reports also recently emerged that a number of officers had exchanged racist text messages.
At a news conference on Thursday, Mayor Lee said he hoped the resignation would help "to heal the city".
The mayor, who until now had supported Mr Suhr, added: "The progress we have made has been meaningful but it hasn't been fast enough, not for me and not for Greg, and that's why I have asked Chief Suhr for his resignation."
He named Toney Chaplin as acting police chief.
The black woman, 27, was shot and killed earlier on Thursday in the city's Bayview area.
Police said one of their patrols approached her as she sat in a car that had been reported stolen.
The woman allegedly tried to drive off and then crashed into a nearby vehicle.
There was no immediate indication that she had a weapon or had tried to run down a police officer before the shooting, the city authorities said.
A 34-year veteran of the San Francisco PD, Greg Suhr was once a popular and professional policeman.
"Greg was always respectful, always a servant of the community," recalled London Breed, who first encountered Suhr when the latter was a young narcotics officer working the beat. Both men would go on to greater things: Suhr to police chief, Breed to president of the local Board of Supervisors.
But for Suhr there were missteps along the way - among them a demotion from deputy chief after a female friend told him she had been assaulted by her boyfriend and he failed to file a police report.
Reflecting on Suhr's resignation, London Breed said he hoped the city would now come together so that everyone would feel safe in their communities.
The job of reforming the police department now rests with Greg Suhr's former deputy Toney Chaplin - another insider, with 26 years of service under his belt.
In April, five people went on a hunger strike, demanding Mr Suhr be sacked. They ended their strike last week.
Mr Suhr, a veteran officer, was appointed city police chief in 2011.
There are more than 1,000 fatal shootings by police in the US each year, and those killed are disproportionately African-American.
