Clashes broke out at an anti-capitalist May Day march in Seattle on Friday when protesters threw rocks and wrenches at police, who responded by lobbing stun grenades and shooting pepper balls, according to police and media reports.
Violence marred May Day demonstrations elsewhere along the West Coast, but other rallies staged across the country appeared to pass largely without incident. Seattle saw some of the worst trouble.
"This is no longer demonstration management, this has turned into a riot," Seattle Police Captain Chris Fowler said in a statement.
At least three officers were injured, two seriously, and at least 15 people were arrested during the clashes, the police department said on its Twitter account.
Footage uploaded on Twitter on Friday evening showed protesters scattering as police clad in riot gear threw the concussive devices into a crowd.
Police said demonstrators had damaged at least two dozen vehicles and set alight garbage. Images on social media also showed demonstrators smashing store windows.
The Seattle Times newspaper said the protesters had gathered around 6 pm local time after a peaceful march on immigration.