Two Seattle police officers were injured and at least seven people arrested on Sunday after a group of unruly demonstrators hurled rocks, bottles and Molotov cocktails and broke windows, authorities said.
Seattle Mayor Ed Murray blamed Sunday's trouble, which he termed "senseless violence," on a "different crowd" from those attending a peaceful May Day immigration march earlier, but he did not give details.
Protesters gather on May 1 each year to focus attention on labor and immigration issues, but demonstrators in cities across the United States also used the occasion to rally against police violence.
The Seattle police department said in a series of posts on Twitter that one officer had suffered a laceration to the head or face as protesters hurled Molotov cocktails.
After making arrests in several places, police said they had contained one group in a parking lot and were slowly allowing small groups to disperse, but were maintaining a presence in the area.
The Seattle Times newspaper reported that police were using "blast balls" to disperse a crowd throwing rocks and bottles, and breaking windows, in a downtown neighborhood.
Earlier, the paper reported that police had used pepper spray several times to break up throngs of demonstrators.
The unrest mirrored violence at a Seattle May Day march last year when crowds threw bottles and wrenches at police, who responded with pepper spray and flash grenades.
Sixteen people were arrested and three officers hurt in that violence, which similarly erupted after a day of peaceful demonstrations to focus on workers' rights, immigration issues and police treatment of minorities.
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(Reporting by Dan Whitcomb and Chris Michaud; Editing by Clarence Fernandez)