One officer was hurt by a thrown brick.
Police said the 23-year-old man was armed with a handgun, but Assistant Chief Bill Jessup told the Milwaukee Journal Sentinel that it wasn't immediately clear whether the man had pointed a gun or fired at the officer yesterday night.
They described the man as a suspect, but didn't say what led to the traffic stop.
Police with shields and helmets moved slowly into an intersection after 11:00 PM (local time), telling a crowd of about 50 people to disperse.
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Protesters threw rocks and other debris at police, who held up their shields. At least two bus shelters had been thrown into the street, with their glass shattered.
Protesters also began throwing objects at a business a half-block from the intersection. A nearby traffic light was bent over.
Police tweeted that shots had been fired and arrests were being made.
It was at least the second confrontation at the intersection, following an earlier standoff involving more than 100 people pushing against 20 to 30 officers.
The newspaper also reported that one of its reporters was shoved to the ground and punched.
The Police Department tweeted that one officer was taken to a hospital after he was struck by a brick thrown through his squad car window.
Police also tweeted that a gas station had been set on fire. They said firefighters couldn't extinguish the blaze because gunshots were being fired.
A Milwaukee police spokesman didn't immediately respond to messages from The Associated Press seeking comment.
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