The trouble on Sunday night flared after initially peaceful vigils by small groups of demonstrators in the city's Sherman Park neighborhood, where Sylville K. Smith, 23, was killed by a black officer after fleeing a traffic stop on Saturday.
Authorities have said Smith was carrying a stolen handgun which he refused to drop when he was killed. Footage from the officer's body camera has not been made public.
Officers wearing helmets and body armor moved to disperse the crowds after shots rang out on Sunday night. One shooting victim was taken to a hospital, police said, but it was not immediately clear if the injured person was a protester.
One police officer was hospitalized after a rock smashed a patrol car windshield, the department said. Another squad car was damaged, and officers made multiple arrests.
Wisconsin Governor Scott Walker had activated the National Guard on Sunday in case more trouble flared over Smith's death, the latest demonstrations over the use of force by U.S. police against minorities. But despite the violence, police said the guardsmen were not called in.
Milwaukee Mayor Tom Barrett has said any decision to deploy the troops, which are under the dual control of the federal and state governments, would come from the police chief.
Police forces are on alert after deadly ambushes this summer. Five officers were slain by a sniper in Dallas last month as they guarded an otherwise peaceful protest against police killings of black people. A gunman killed three officers in Baton Rouge, Louisiana, less than two weeks later.
'WITHIN LAWFUL BOUNDS'
Policing in Milwaukee has come under scrutiny since 2014, when a white officer killed Dontre Hamilton, a mentally ill, unarmed black man, in an incident that sparked largely peaceful demonstrations.
Police say Smith was stopped on Saturday afternoon for behaving suspiciously and that he then fled on foot between two homes. During the first night of protests over his death, 17 people were arrested, and four officers were injured.
Aiming to reassure the community that the police acted properly, Chief Edward Flynn said on Sunday that video from the officer's body camera showed Smith had turned toward him with a gun in his hand.
Flynn told a news conference that a silent video of the incident appeared to show the officer acting within the law. Because the audio was delayed, he said, it was not clear when the officer fired his weapon.
"I'm looking at a silent movie that doesn't necessarily tell me everything that will come out in a thorough investigation," Flynn said. "Based on what I saw, didn't hear, don't know what the autopsy results are going to be, (the officer) certainly appeared to be within lawful bounds."
Barrett said Smith did not drop the gun as ordered before he was shot.
The mayor said Smith had a lengthy arrest record, and officials had earlier said he was carrying a stolen handgun loaded with 23 rounds of ammunition when stopped.
Before the clashes broke out on Sunday evening, about 200 people had gathered to light candles near where Smith was killed. A few officers looked on as faith and community leaders implored protesters to restrain their anger.
"We are not ignorant and stupid people," one pastor told the crowd, echoing a feeling among many of the city's African-Americans that they are systematically mistreated.
"Every single person needs to be looked upon as human beings and not like savages and animals."
Several of Smith's sisters addressed the crowd on Sunday evening, saying their brother did not deserve to be shot.
"My brother was no felon," one of them, Kimberly Neal, said as she wept.
"My brother was running for his life," said Neal, 24. "He was shot in his back."