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 p.m. local time after a peaceful march on immigration.
Pepper spray and 'flashbangs' were also used in Portland after some protesters threw objects at officers and tried to force their way onto a bridge, the city police department said on its Twitter account. One officer was injured, it said.
In Oakland, California, windows of businesses and banks were smashed, and car dealerships vandalised during a protest involving several hundred people, according to local media and police.
Protesters annually assemble on May 1, marking International Labor Day, as a day to focus attention on labor and immigration issues. Demonstrators in cities across the country also used the occasion to rally against police violence.
In Baltimore, demonstrations were peaceful and even celebratory on Friday after prosecutors brought charges against all six officers involved in the arrest of Freddie Gray, the 25-year-old black man who died of spinal injuries suffered while in police custody earlier this month.
Gray's death has become the latest flashpoint in a national outcry over excessive force used against African-Americans and other minority groups by the white-dominated U.S. law enforcement establishment. It set off riots in Baltimore on Monday.