The southeastern Wisconsin city became the nation's latest flashpoint in a summer of racial unrest.
Rocks, bottles, and fireworks thrown at police officers.
All three officers were stable and had injuries not considered to be life-threatening.
Three police officers were injured, and 18 people were arrested as a result of a riot in Seattle involving explosives, bottles and rocks, authorities said.
The gunfire erupted about 11:30 a.m. at Mission Market during a dispute on the grounds, according to Police Chief William McManus.
Officers came under fire in one of several shootings across the city during another violent weekend.
At least 18 people were shot, including four killed, as gunfire erupted in several places around the city overnight, authorities said Sunday.
A bizarre crash assistance story from Koblenz.
Police said at least three shooters may have been involved, though no arrests were immediately announced.
Some demonstrators filled pool noodles with nails and placed them in the road, causing extensive damage to a patrol vehicle, police said.