Crowds clashed with police during May Day marches in several US west coast cities late on Friday, as officers responded with stun grenades and pepper spray, police and media said.
Anti-capitalist protesters hurled wrenches and rocks at officers in Seattle, police said. Demonstrators in Oakland, California, and several other cities, rallied against a series of police killings of unarmed black, local media reported.
Footage on social media showed protesters smashing shop windows in Seattle and crowds scattering as police clad in riot gear threw in "flashbang" grenades. Demonstrators set fire to garbage and damaged at least two dozen vehicles, police said.
"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 16 people were arrested, Seattle's police department said on its Twitter account.
Several hundred protesters snaked through the streets of Oakland for hours on Friday night after a day of peaceful protests.
More than 100 windows at businesses, restaurants and banks along the route were smashed, and several people were taken into custody overnight. At least one vehicle was burned and others damaged on the lot of a local car dealership.
Oakland police did not immediately respond to request for comment.
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.
Protesters annually assemble on May 1 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.
Many rallies proceeded largely without major incident.
In Baltimore, demonstrations were peaceful and even celebratory 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 US law enforcement establishment. It set off riots in Baltimore on Monday.
