A total of 45 people were arrested and 21 police officers injured on Saturday after a "Black Lives Matter" protest turned violent in the western U.S. city of Seattle, local police said.

"As of 10 p.m. (0500 GMT): Police have made 45 arrests in connection with today's riot in the East Precinct. 21 officers sustained injuries after being struck by bricks, rocks mortars/other explosives," the Seattle Police Department tweeted late on Saturday.

"Most officers were able to return to duty. One was treated at a hospital for a knee injury," it added.

Earlier, a group of protesters set fire and damaged a portable trailer and the construction site for a youth detention facility, the police said.

The United States has been gripped by nationwide demonstrations against racial discrimination following the death of African American George Floyd in May.