Authorities in the US state of Wisconsin were urging residents to remain calm and off the streets night, some 24 hours after violent protests rocked Milwaukee following a fatal shooting by police.

Mayor Tom Barrett told reporters that he hoped not to deploy the 125 members of the Wisconsin National Guard who had been activated earlier in the day to prevent a repeat of the arson, rock-throwing and shootings that occurred the previous night.

"If you love your son, you love your daughter, you love your grandchild, tell them to stay away from this area," Barrett said during a press conference Sunday afternoon.

While an uneasy calm had returned to the affected neighborhood, he said that the city still faced "a very volatile situation".

Violence broke out overnight Saturday to Sunday as an angry crowd of at least 200 took to the streets, torching at least six businesses, including a BP gas station and auto parts store, police said.

Individuals attending the rally fired dozens of shots, apparently in the air, authorities said.

Milwaukee police posted images on Twitter showing damage to an armored rescue vehicle caused by people throwing rocks and concrete and firing bullets.

Gun in hand

Barrett emphasized that the individual killed by police -- a 23-year-old local man named Sylville Smith -- had been holding a gun, which was clearly visible in a still shot taken from the body camera of a police officer on 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 violence in the Midwestern city comes after 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.

The shooting occurred when two police officers stopped a car Saturday and two suspects inside, including Smith, fled on foot.

"During the foot pursuit, one officer shot one suspect, armed with a semiautomatic handgun" and the suspect then died at the scene, the Milwaukee Police Department said in a statement.

Milwaukee Police Chief Edward Flynn told reporters Smith had ignored an order to drop his weapon and that the entire incident lasted no more than 25 seconds.

Police said earlier that the handgun he was carrying had been stolen during a burglary in March.

Flynn added that the unnamed officer who fired on Smith was black. Fearing for his safety, he was now staying with relatives out of town.

The officer had been placed on administrative leave, as is standard in such situations, Flynn said.

US President Barack Obama was updated on the situation by a senior advisor, who spoke with Barrett to offer the administration's support for local authorities, the White House said.

'Tremendous restraint'

"What you saw last night was tremendous restraint by our police officers. Not a single shot was fired" by police, Barrett said.

One officer was taken to the hospital after being hit in the head by a brick thrown through a squad car window. Protesters also broke the windows of an unoccupied squad car and torched another, police said.

The mayor said four officers had been taken to the hospital, but all had been released.

Flynn said one 16-year-old female had been shot -- possibly by a stray bullet -- and suffered non-life-threatening injuries.

He said 17 arrests had been made overnight, and that all of those arrested had prior criminal records.

"We cannot have a repeat of what happened last night. Some additional forces have been mobilized and put together. A plan is in place. A plan that's fluid," Milwaukee County Sheriff David Clarke told a press conference.

Wisconsin Governor Scott Walker activated the state's National Guard earlier in the day after discussions with Clarke and Barrett, among others.

"There are a lot of really, really good people who live in this area... who can't stand this violence and they want order restored," Barrett told reporters, as he pleaded for calm.

He praised what he said were the scores of local people who came to the neighborhood Sunday to help clean up.

The United States has been on edge for weeks following shootings targeting police officers in several cities, among them Dallas and Baton Rouge, Louisiana.

Those attacks followed anger among the black community over the fatal shooting of African American suspects by white police in Minnesota and Louisiana.

In Milwaukee, local official Khalif Rainey, who represents the area where the disturbances took place, called the violence a "warning cry."

"This entire community has sat back and witnessed how Milwaukee, Wisconsin, has become the worst place to live for African-Americans in the entire country," he was quoted as saying.