One person has been shot at a Milwaukee protest as tense skirmishes erupted for a second night following the police shooting of a black man.
Officers used an armoured vehicle to retrieve the injured victim and take them to a hospital.
About two dozen officers in riot gear confronted about 150 people who blocked a junction near the fatal shooting on Saturday afternoon as more police arrived.
Police moved in to try to disperse the crowd and warned of arrests after protesters threw bottles and rocks at officers and shots were fired.
Earlier, police chief Edward Flynn said the man whose death sparked Saturday night's rioting was shot after he turned towards an officer, who was also black, with a gun in his hand.
Wisconsin governor Scott Walker put the National Guard on standby for any repeat of violence. Protests were peaceful for most of Sunday evening before the confrontation after 11pm local time.
Mr Flynn said the shooting was still under investigation and authorities were awaiting post-mortem test results, but based on the silent video from the unidentified officer's body camera, he "certainly appeared to be within lawful bounds".
Mayor Tom Barrett said a still image pulled from the footage clearly showed a gun in 23-year-old Sylville Smith's hand as he fled a traffic stop on Saturday.
"I want our community to know that," Mr Barrett said. But he also called for understanding for Smith's family.
"A young man lost his life yesterday afternoon," he said. "And no matter what the circumstances are, his family has to be hurting."
Mr Flynn would not identify the officer who shot Smith but said he was black. The police chief said he was not sure what prompted the stop but described Smith's car as "behaving suspiciously".
After watching the officer's body camera footage, Mr Flynn said the entire episode took about 25 seconds, from the start of the traffic stop until shots were fired. He said Smith ran "a few dozen feet" and turned towards the officer while holding a gun.
"It was in his hand. He was raising up with it," the chief said. He said the officer had told Smith to drop the gun but he did not. It was unclear how many rounds the officer fired. Smith was hit in the chest and arm, Mr Flynn said.
Mr Walker activated Wisconsin's National Guard, and 125 guard members reported to local armouries to prepare for further instructions. Mr Flynn said 150 police officers specially trained in managing big protests had also been mobilised.
Six businesses were burned in the unrest that spilled past midnight on Sunday. Seventeen people were arrested and four officers were hurt from flying concrete and glass, although all of them were released from hospital.
Milwaukee alderman Khalif Rainey, who represents the neighbourhood that erupted, said the city's black residents were "tired of living under this oppression".
"Now this is a warning cry. Where do we go from here? Where do we go as a community from here?" he said.
Milwaukee County sheriff David Clarke said Smith had been arrested 13 times and online court records showed a range of charges against him, many of them misdemeanours.
The Milwaukee Journal Sentinel said Smith was also charged in a shooting and later with pressuring the victim to withdraw evidence that identified Smith as the gunman. The charges were dropped because the victim recanted the identification and failed to appear in court, chief deputy district attorney Kent Lovern told the newspaper.
Smith's sister said the family wanted prosecutors to charge the officer who shot him.
Kimberly Neal, 24, spoke as supporters surrounded her at the vigil as she held a bouquet of blue balloons.
She asked people for donations for his burial.
Asked about the violence on Saturday night, Ms Neal said: "People stuck together and they are trying to stand up (for their rights)."
The anger at Milwaukee police is not new and comes as tension between black communities and law enforcement has ramped up across the nation, resulting in protests and the recent ambush killings of eight officers in Baton Rouge, Louisiana, and Dallas.
Nearly 40% of Milwaukee's 600,000 residents are black and they are heavily concentrated on the north side.
Milwaukee was beset by protests and calls for police reform after an officer shot dead Dontre Hamilton, a mentally ill black man, in 2014.
In December, the US Justice Department announced it would work with Milwaukee police on changes.
Critics said the police department should have been subjected to a full Justice Department investigation like the one done in Ferguson, Missouri, after the killing of black teenager Michael Brow, 18, in 2014 sparked violence there.
The officer involved in the most recent Milwaukee shooting was 24 and has been on the force for three years, according to the department.
There was no repeat of widespread destruction of property, however.
Television footage showed police forming a line and using shields to deflect objects and one officer fell to the ground after apparently being hit by one of the missiles and was moved away by colleagues.
Police also said an injured officer was taken to hospital after a rock broke the windscreen of a squad car.
Earlier, video taken from a media helicopter showed a small group of protesters running through the streets, picking up orange construction barriers and hurling them out of the way. Police posted on Twitter three locations where they said shots were fired.