Five Seattle police officers were injured and at least nine people arrested on Sunday night, after unruly demonstrators hurled projectiles and Molotov cocktails and broke windows, authorities said.
Protesters gather every year on May 1 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.
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Mayor Ed Murray blamed the "senseless violence" in Seattle on a "different crowd" from those who had attended an earlier peaceful May Day immigration march, however.
It was "deeply regrettable that in a city that goes to incredible lengths to respect First Amendment rights, there are some who disregard our values and engage in senseless acts of violence and property destruction," he said later, referring to free speech protected by the U.S. Constitution.
In posts on social media website Twitter, the Seattle police department said one officer had suffered a cut to the head as protesters hurled Molotov cocktails, another was hit by a rock, and a third officer was bitten, apparently by a protester.
Injuries suffered by two more officers were not detailed.
"Nobody has been seriously injured," police chief Kathleen O'Toole said after the unrest subsided, adding, "Once assaults started and property damage started, we took action. It's that simple."
Police said they made arrests in several places and restrained one group in a parking lot, before allowing some groups to disperse, but would maintain a presence in the area.
They charged three people with assault, one with destruction of property and five with obstruction of justice. Eight men ranging in age from about 20 to 32 were charged, along with a teenage girl.
Police used "blast balls" to disperse a crowd throwing rocks and bottles and breaking windows in a downtown neighborhood, the Seattle Times newspaper reported, after they 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.