MILWAUKEE (AP) -- Wisconsin's governor put the National Guard on alert Sunday in case of another outbreak of violence in Milwaukee, after a deadly police shooting touched off a night of arson and rock-throwing in a mostly black neighborhood.
At least four businesses were burned and one police officer was hurt by a thrown brick in the unrest that erupted on the city's north side Saturday night a few hours after the killing of a man authorities say was armed and fleeing a traffic stop.
The races of the man and the officer who shot him weren't immediately disclosed, but a Milwaukee alderman called the resulting melee a warning from black residents "tired of living under this oppression."
Gov. Scott Walker activated the National Guard, saying it would be position to help upon request. He called for "continued peace and prayer."
A Guard spokesman, Lt. Col. Gary Thompson, said 125 soldiers were being notified to gather at their local armories and await instructions.
On Sunday morning, about three dozen volunteers swept up glass and filled trash bags with rocks, bricks and bottles at the intersection where a gas station burned to the ground and bus shelters were knocked over. One volunteer picked up a bullet casing and handed it to police.
Darlene Rose, 31, said that she understands the anger that fueled the violence, but that it doesn't help.
"I feel like if you're going to make a difference, it's got to be an organized difference," Rose said. "The people that came and looted, you're not going to see them here today."
Three protesters were arrested in the violence.
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.
The protesters on Saturday night were largely black, and Alderman Khalif Rainey, who represents the district, said the city's black residents are "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?" Rainey said.
Nearly 40 percent 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 and killed Dontre Hamilton, a mentally ill black man, in 2014.
In December, the U.S. Justice Department announced it would work with Milwaukee police on reforms. Police Chief Ed Flynn had asked for what's known as a collaborative reform process after the federal government said it wouldn't pursue civil rights charges against the officer.
Critics said the police department should have been subjected to an investigation of its practices, as was conducted in Ferguson, Missouri, after the killing of black 18-year-old Michael Brown by a white police officer in 2014 touched off violence there.
The weekend shooting in Milwaukee is under investigation. The mayor said the officer was wearing a body camera.
Mayor Tom Barrett said police stopped the 23-year-old motorist for what the mayor described only as "suspicious activity." Police said the man - whose name was not immediately released - had an arrest record and was carrying a gun that had been stolen in a burglary in March.
"There were 23 rounds in that gun that that officer was staring at," the mayor said. "I want to make sure we don't lose any police officers in this community, either."
The unidentified 24-year-old officer was put on desk duty. The officer has been with the Milwaukee department six years, three as an officer, authorities said.
At one point Saturday evening, as many as 100 protesters massed at 44th Street and Auer Avenue, surging against a line of 20 to 30 officers.
The Milwaukee Journal Sentinel reported that some in the crowd started smashing a squad car's windows. Another police car was set on fire. The newspaper reported that one of its reporters was shoved to the ground and punched.
In addition to the gas station, a bank, an auto parts store and a beauty supply shop were burned. Firefighters held back from the gas station blaze because of gunfire.
Associated Press writer Kyle Potter contributed to this report from Minneapolis.