OAKLAND, United States - More than a dozen people were arrested in the Californian city of Oakland after a May Day protest turned violent, police said Saturday.
Between 300 and 400 people gathered in the downtown area on Friday evening but within hours several demonstrators had become aggressive, the Oakland Police Department said.
"Individuals within the groups committed multiple acts of vandalism," police said in a statement.
"These acts of vandalism included broken windows, graffiti, small fires and failure to disperse." Demonstrators had gathered to mark May Day, a traditional day of rallies for workers rights, but this year's focus was on the issue of police brutality.
Protests erupted across the United States this week following the death of Freddie Gray, a 25-year-old Baltimore man who died from injuries sustained while in police custody.
Six officers were charged in the case on Friday, but protests still erupted in Baltimore after demonstrators defied a 10pm curfew.
Separately, three police officers were injured in Seattle on a Friday evening demonstration and 16 people were arrested, according to police.
Demonstrations have also taken place in recent days in other US cities including New York, Philadelphia, Washington and Boston.
Gray has become the latest name in an ongoing national debate over whether US police are too quick to use violence against unarmed black males.
For Oakland residents, the issue brings back memories of an infamous case in which rioting broke out after a white police officer fatally shot 22-year-old Oscar Grant in the back as he lay face down at a transit station on New Year's Day in 2009 - a killing bystanders caught on video.