Simmering anger over the fatal shooting of a man by police erupted in violence on Milwaukee's north side, with protesters skirmishing with officers over several hours and setting fire to at least four businesses in an outburst the mayor says was fed by social media.

The uprising that broke out on Saturday evening didn't subside until after midnight, after Mayor Tom Barrett and other city leaders appeared at a news conference to plead for calm. Police said three people were arrested, and one officer was hurt by a brick thrown into a squad car.

The triggering event came on Saturday afternoon, when a man fleeing police after a traffic stop was shot and killed. Police said the man was armed, but it wasn't clear whether he was pointing the gun or aiming it at officers. Barrett said the man was hit twice, in the chest and arm. Neither his race nor the officer's was immediately released, nor were they identified.