City leaders have appealed for calm in Milwaukee after violence erupted when police shot dead an armed man during a chase.
More than 100 protesters took to the streets of the city's north side on Saturday night after news of the shooting spread, erupting in clashes with police. Several businesses were also set alight and an officer was injured by a brick.
At a news conference later, Mayor Tom Barrett said the situation appeared to be calming after riotous scenes in which protesters skirmished with police, torching a squad car and throwing a brick through the window of another. Police mounted at least two efforts to push the protesters from an intersection at the heart of the violence.
The unrest began several hours after the 23-year-old man trying to flee from police after a traffic stop was shot and killed.
Police said the man - who had a criminal record - was armed with a stolen handgun, but it was not clear whether he was pointing the gun or aiming it at officers.
Mr Barrett said the man was hit twice, in the chest and arm. Neither his race nor the officer's was immediately released, nor were they identified.
The shooting was being investigated by the state. The officer was wearing a body camera, Mr Barrett said.
At least three people were arrested in an uprising that Mr Barrett said was driven by social media messages instructing people to congregate in the area.
"We have to have calm," Mr Barrett said. "There are a lot of really good people who live in this neighbourhood."
Milwaukee Common Council president Ashanti Hamilton echoed the mayor's plea for help restoring order.
"We understand the frustration people feel with the police community nationally. ... We have to go through the process of finding justice, but we have to be able to restore order to these neighbourhoods," Mr Hamilton said.
"Please participate in restoring order to these neighbourhoods."
Mr Barrett said the man who died was stopped by police for "suspicious activity". Police said he was carrying a gun that had been stolen in a March burglary in suburban Waukesha.
"This stop took place because two officers ... saw suspicious activity," Mr Barrett said. "There were 23 rounds in that gun that that officer was staring at. I want to make sure we don't lose any police officers in this community, either."
Police with shields and helmets moved slowly into the road where the protesters had gathered telling a crowd of about 50 people to disperse. Some threw rocks and other debris at police, who held up their shields. Several businesses were set alight during the unrest.
An earlier stand-off involved more than 100 people pushing against 20 to 30 officers. Officers got in their cars to leave at one point and some in the crowd started smashing a squad car's windows. Another police car was set on fire.
The 24-year-old officer who shot the man has been placed on administrative duty. The officer has been with the Milwaukee police department for six years, three as an officer.