Protesters threw rocks and fired guns in a second night of violence in a tense neighborhood of Milwaukee following the deadly police shooting of an armed black man.
Police in riot gear moved in on the Sherman Park neighborhood in the midwestern city around 11:00 pm on Sunday (0400 GMT Monday) to disperse an angry crowd and restore order, eager to avoid a repeat of previous violence involving law enforcement that has shaken the nation.
Officers at one point used an armored vehicle to rescue a shooting victim and rush the person to the hospital.
A police officer was hospitalised to be treated for an injury after protesters threw a rock that broke the windshield of a squad car.
An unspecified projectile was thrown at another officer, who was protected by a helmet, according to police.
One squad car was damaged by thrown bricks, rocks and glass bottles.
"Officers continue to have rocks thrown at them as they work to disburse small, disorderly groups in the area around Sherman and Burleigh," police said on Twitter, adding that armored vehicles were deployed to protect officers.
Multiple arrests were made, it said without giving further details.
Some 125 members of the Wisconsin National Guard were activated and placed on standby early Sunday to prevent a repeat of the arson, rock-throwing, and shootings that occurred the previous night.
But they were not called to the areas of the latest disturbances.
Some locals reacted with shock at the degree of violence.
"I've lived here for 30 years, and I've never seen anything like this in my life," Dominic Lebourgeois told The New York Times.
"I think it's crazy, it's ludicrous."
On Saturday, an angry crowd of at least 200 people took to the streets, torching at least six businesses, including a gas station and auto parts store that were destroyed, police said.
Individuals attending the rally fired dozens of shots, apparently in the air.
Violence broke out following the fatal shooting of a local man, Sylville Smith, 23.
Even though Smith was said to be armed and had a lengthy police record, the shooting, and its aftermath bore chilling echoes of a series of violent incidents involving law enforcement.
Several police officers have been targeted and shot dead across the nation in recent weeks, including five in Dallas, following an outcry over the deaths of unarmed African Americans at the hands of police.
"I lost my brother. I can't get him back. Never. Never. That's pain. That's real hurt," Smith's younger sister Sherelle said in an emotional plea during a vigil."
"I can't look my brother in the eye and say, 'I love you.' I didn't even have a Facebook to tell my brother I love him."
The shooting began when a car carrying Smith and another individual was stopped by two police officers. Smith and the other suspect fled on foot. Police then shot and killed Smith after he failed to drop a gun, according to the authorities.
Mayor Tom Barrett emphasised that Smith had been holding a semiautomatic handgun, which was clearly visible in a still shot taken from the body camera of a police officer on the scene.
"That still photo demonstrates, without question, that he had a gun in his hand. And I want our community to know that," Barrett said.
The unnamed officer who fired on Smith was black, according to Milwaukee Police Chief Edward Flynn. Fearing for his safety, he was now staying with relatives out of town.
The officer has been placed on administrative leave, as is standard in such situations.
Barrett, pleading for calm, warned that the city still faced "a very volatile situation".
"What you saw last night was tremendous restraint by our police officers. Not a single shot was fired" by police, Barrett said about Saturday's outburst, which resulted in 17 arrests.
One officer was taken to the hospital after being hit in the head by a brick thrown through a squad car window.
The mayor said four officers had been taken to the hospital, but all had been released.
A 16-year-old girl was shot, possibly by a stray bullet, and suffered non-life-threatening injuries.