At least two efforts to push protesters back from the main intersection in the north of the city where the majority of the unrest occurred.
The violence came after an unnamed 23-year-old man was shot dead while trying to flee from police at a traffic stop.
Police said the man – who had a criminal record – had a stolen handgun but it was not clear whether he was brandishing it when he was shot.
Neither his race nor that of the officer who fired the fatal shot have been identified.
Mr Barrett told a news conference that the shooting was being investigated and the officer was wearing a body camera.
The unnamed 24-year-old officer has been put on administrative duty. He has been at the Milwaukee police department for six years.
Milwaukee has become the latest US city to be hit by violence after police shot an armed man dead during a chase.
City officials have appealed for calm after more than 100 protesters took to the streets after news of the shooting spread.
Protesters clashed with officers while several businesses – including a petrol station – and a police car were set alight.
An officer was injured after a brick was thrown through the window of another police car.
Mayor Tom Barrett said the situation appeared to be calming as police mounted
Mr Barrett said: This stop took place because two officers saw suspicious activity.
A petrol station burns while more than 100 people protest on the streets of Milwaukee after another police killing (AP)
There were 23 rounds in that gun that that officer was staring at. I want to make sure we don’t lose any police officers in this community, either.
Milwaukee Common Council president Ashanti Hamilton echoed Mr Barrett’s call for calm.
He said: We understand the frustration people feel with the police community nationally. We have to go through the process of finding justice, but we have to be able to restore order to these neighbourhoods.
Please participate in restoring order to these neighbourhoods.At least three people have been arrested during the latest wave of violence in Wisconsin which follows recent unrest over the shooting of several black men in separate incidents across the country last month.
Alton Sterling in Baton Rouge, Louisiana, Philando Castile in St Paul, Minnesota and Delrawn Small in Brooklyn, New York were all killed by police officers in a three-day period last month sparking mass protests across the country.
The Black Lives Matter movement, which protests against the perceived brutality with which US police officers treat black suspects, has organised many of the protests but has urged its supporters to protest peacefully.