A US governor activated the National Guard in Wisconsin today to assist should further rioting erupt in a predominantly black Milwaukee neighborhood where police killed an armed suspect.
A police officer shot and killed an armed suspect who took flight after a traffic stop, authorities said.
The 23-year-old suspect, who had a lengthy arrest record, was carrying a stolen handgun loaded with 23 rounds of ammunition when police pulled over the vehicle for unspecified "suspicious activity".
A second suspect who fled from the vehicle was quickly taken into custody.
Later, a crowd of more than 100 people in the predominately African-American area of the city where the shooting occurred, hurled rocks as police officers in riot gear attempted to disperse the protesters.
Authorities said gunshots were fired during the disturbance. The windows of at least two squad cars were smashed, and one officer sitting inside one of the vehicles was hit in the head with a brick.
Protesters set a police car ablaze and fires broke out at a petrol station, a car parts shop and at least three other businesses, officials and local media reported.
The Sherman Park neighborhood, which has a reputation for poverty and crime, appeared calm at midday today after the overnight rioting by demonstrators angered by the police killing. At least three people were arrested.
Police presence in the area was low, with some residents milling around and others cleaning up debris.
Wisconsin Governor Scott Walker announced the National Guard activation - the reserve military - after a request from Milwaukee County Sheriff David Clarke, who met with Mr Walker and Wisconsin National Guard Adjutant General Donald Dunbar.
Sheriff Clarke, an African-American, has in the past advised residents to arm themselves against criminals. The sheriff, who spoke last month at the Republican National Convention, has publicly opposed the Black Lives Matter movement.
Mr Walker said the Guard would be "in a position to aid local law enforcement upon request".
Police violence against black men and women has set off intermittent, sometimes violent protests in US cities from Ferguson, Missouri, Baltimore, New York, Baton Rouge, Louisiana, and St. Anthony, Minnesota, in the past two years.
The outrage has given rise to Black Lives Matter and touched off a national debate over race and policing in the United States.
There also have been deadly ambushes of police this summer. Five officers were slain by a sniper in Dallas in July as they provided security at an otherwise peaceful protest of police killings. Three officers were killed by a gunman in Baton Rouge less than two weeks later.
