Dozens were arrested and many police injured in clashes around Seattle's biggest Black Lives Matter protest in weeks on Saturday, police said, with a renewed energy sparked by violent clashes between activists and federal agents in nearby Portland, Oregon.

Police said officers used non-lethal weapons in attempts to disperse the thousands of marchers in the late afternoon after some protesters set fire to the construction site for a King County juvenile detention facility and courthouse.

By 10 p.m.(0500 GMT), police had "made 45 arrests in connection with today's riot in the East Precinct," the Seattle Police wrote in a Twitter post.

It said "21 officers sustained injuries after being struck by bricks, rocks mortars/other explosives. Most officers were able to return to duty. One was treated at a hospital for a knee injury."

(With input from the agencies)