A police car with broken windows is seen in a photograph released by the Milwaukee Police Department after disturbances following the police shooting of a man in Milwaukee, Wisconsin, US, August 13, 2016. [Photo/VCG]

MILWAUKEE - A Milwaukee policeman whose fatal shooting of a suspect sparked overnight rioting in the US Midwestern city appeared to have acted lawfully after the man turned toward him with a gun in his hand, Police Chief Edward Flynn said on Sunday.

Wisconsin Governor Scott Walker activated the state's National Guard in case of more violence in response to the death of Sylville K. Smith, 23, who was shot while trying to flee from an officer who had stopped his car on Saturday.

In a scene Mayor Tom Barrett described as "unlike anything I have seen in my adult life in this city," crowds gathered after the shooting and then turned violent during the night in the city's Sherman Park neighborhood.

Gunshots were fired, six businesses were destroyed by fire and police cars damaged before calm was restored in the neighborhood, which has a reputation for poverty and crime.

Seventeen people were arrested in the disturbances, Flynn said. Four police officers were treated for injuries and released from hospitals.

Flynn, appearing at a news conference with Barrett, said the officer who fired the fatal shot was black and media reports also identified Smith as black.

He said a silent video of the incident appeared to show the officer acting within lawful bounds in shooting Smith. Flynn said the video from the officer's body camera showed Smith turning toward the officer with a gun in his hand.

Flynn said the officer stopped Smith's vehicle because he was behaving suspiciously and then had to chase him several dozen feet on foot into an enclosed space between two houses.

The mayor told the news conference that Smith did not drop the gun as ordered before he was shot.