Protesters in northern California hurled objects at police, who responded by firing gas for a second consecutive night, as demonstrations continued over the death of a black man after he was put in a chokehold by a police officer in New York.
A crowd of about 750 demonstrators swarmed freeway overpasses at two locations in Berkeley, near San Francisco, prompting police to move in to clear the roadways and arrest several people.
The California Highway Patrol's Golden Gate division said on its Twitter feed that gas was fired after some demonstrators began hurling objects it described as unspecified "explosives," and arrests had been made, but exact numbers were not available.
Some demonstrators said on Twitter that police had fired rubber bullets, but this could not be confirmed.
West Coast cities had braced for trouble after clashes in nearby Berkeley and Seattle, Washington on Saturday. Protesters in New York and other cities have staged demonstrations every day since a grand jury's decision on Wednesday not to bring criminal charges against the white police officer whose chokehold contributed to the man's death in New York in July.
Saturday's looting and rock-throwing on the West Coast contrasted with the mostly peaceful demonstrations that have taken place elsewhere. New York was quieter over the weekend.
The killings of Eric Garner in New York and Michael Brown, an unarmed black teen, in Ferguson, Missouri, have highlighted the strained relations between police and African-Americans and rekindled a national debate over US race relations.
The decision by a grand jury not to return an indictment in Brown's killing ignited two nights of arson and rioting in the St. Louis suburbs.
Earlier on Sunday evening hundreds of protesters began marching down a main thoroughfare in Berkeley after massing on the campus of the University of California.
Officer Jenn Coats of the Berkeley Police Department said the crowd, estimated at 500 to 600, was peaceful.
On Saturday evening, what had begun as a peaceful march ended in an extended confrontation between demonstrators and police, resulting in six arrests, damage to local businesses and a minor injury to a police officer.
"There were definitely a group of people that were intent on violence," Coats said.
In downtown Seattle, about 200 people gathered on Sunday evening, a day after a demonstration drew more than 1,000 protesters, with some throwing rocks and attacking police in clashes that resulted in seven arrests. Their numbers had dwindled to about 60 by 10 p.m. with only one arrest.
New York Police Commissioner Bill Bratton said on Sunday the department's internal investigation into Garner's death could last four months. He said he would review the results to decide if officers involved in Garner's arrest had violated department policy. The Justice Department is doing its own investigation.
In Chicago, church-affiliated protesters marched through the city, carrying signs and chanting, "I can't breathe" and "Hands up, don't shoot," television news footage showed.
Protesters in Miami blocked a portion of Interstate 195 on Sunday afternoon, clogging traffic to the Art Basel show in Miami Beach, CBS-TV Miami reported.
The outcry over the recent killings surfaced in NFL stadiums as well. Detroit Lions running back Reggie Bush was among several players donning pre-game practice jerseys reading "I can't breathe," Garner's dying words.