CNN  — 

Seattle protesters threw large rocks, bottles, fireworks and other explosives at officers Saturday, police said. Others set fire to a portable trailer and a construction site, police said in a series of tweets.

At least 45 people have been arrested on charges of assaulting officers, obstruction and failure to disperse, police said. Twenty-one officers have been injured from having projectiles thrown at them, according to police. Most officers were able to return to duty, the department’s Twitter said. One officer was hospitalized with a leg injury caused by an explosive.

Police described the protest as a riot.

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

The zone known as the CHOP was started by demonstrators calling for justice in the death of George Floyd.

Police cleared the zone on July 1. Three days later, a protester was killed during a demonstration.

CNN’s Samantha Guff, Jennifer Henderson, Madeline Holcombe and Elle Reeve contributed to this report.