The scene of a protest in Seattle Sunday night.
CNN  — 

At least 18 people were arrested in Seattle Sunday night when a riot was declared during an anti-police demonstration in the SODO neighborhood, just south of downtown, according to the Seattle Police Department. Police did not say what charges the arrestees face.

Police said the march, which began in Seattle’s International District around 7 p.m., started peacefully with about 100 protesters accompanied by vehicles following behind with the group, which was headed toward Seattle’s police officer union building. Then “shortly after 10 p.m., someone in the crowd set off a large explosive and attempted to break out a police vehicle window,” police said.

Protesters then began throwing rocks, bottles, and explosives at officers, police said. Video from CNN affiliate KIRO shows what appears to be fireworks exploding near officers as police attempted to disperse the crowd. Officers used pepper spray and blast balls to disperse the demonstrators, police said. KIRO reported that the protest ended around 11:30 p.m.

At least three officers were injured, some struck with explosives and rocks, police said. Seattle Fire Department responded to treat the injured officers; one officer required hospitalization.

Seattle police said the investigation is ongoing and more information will be available later.

Seattle has been the scene of protests over police brutality and systemic racism for weeks, including in a six-block area controlled by protesters after police abandoned their precinct – the Capitol Hill Organized Protest or Capitol Hill Autonomous Zone, known as CHOP.

President Trump has been critical of mayors in major cities, including Seattle, Chicago, Portland, and Atlanta, for what he sees as a failure to keep law and order.

CNN’s Christine Sever and Alta Spells contributed to this report