Dozens of U.S. congressional staff staged a walkout on Thursday to protest decisions by grand juries not to charge white police officers in the killings of unarmed black men in Ferguson, Missouri, and New York City.

The staffers, including members of the Congressional Black Associates group, held a prayer service on the steps of the U.S. Capitol and raised their hands in a reference to the "Hands up, don't shoot" chants that have become a feature of protests around the nation.

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The action was the latest in a series of demonstrations over concerns about the policing of black communities. Some have turned violent, including this week's protests in northern California.

Oakland and neighbouring Berkeley, California, have seen nightly demonstrations all week in response to decisions by two grand juries not to charge white police officers in the killings of Michael Brown in Ferguson and Eric Garner in New York.

On Wednesday night, about 150 protesters, fewer than on previous evenings, left the campus of University of California, Berkeley, and demonstrated without incident before marching south into Oakland, authorities said.

By that point, the protesters had dwindled to about 50 people, some of whom broke windows at a T-Mobile store and a Chase bank branch, according to the city of Oakland. Looting was reported in an area of small businesses at a downtown intersection, the city said.

At one point, a Reuters photographer witnessed a man who demonstrators said was an undercover police officer and who had been marching with them, pointing a pistol at protesters after he and another man were attacked.

Within a minute or two, about 20 uniformed Oakland police officers arrived and detained one of the protesters.

Chief Avery Browne, commander of the California Highway Patrol's Golden Gate Division, said two plainclothes CHP detectives were surrounded by up to 50 demonstrators who ignored orders to back off, despite one of the officers first taking out his baton and identifying himself as police.

"We are extremely cognizant and very sensitive to the display of a gun. It's very upsetting. It's very disturbing to individuals who are attempting to peacefully protest, and we recognise that," Browne told reporters by telephone.

But he said the detective involved told him he had been in fear for his and his partner's lives. "No one has provided any evidence that the officers were inappropriate in what they did," Browne said.

On previous evenings this week in the Bay Area, riot police have fired tear gas and pepper spray to disperse crowds of demonstrators that have at times been hundreds strong, some of whom have thrown stones at the officers.

African-American Congressional staffers and representatives stage a walk out with a "Hands Up, Don't Shoot" pose on the steps of the House of Representatives at the U.S. Capitol to protest the deaths of Michael Brown and Eric Garner, in Washington December 11, 2014.