WASHINGTON/OAKLAND, California (REUTERS) - Dozens of 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 US Capitol and raised their hands in a reference to the "Hands up, don't shoot" chants that have become a regular feature of protests around the nation.

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 man, who demonstrators said was an undercover police officer and who had been marching with them, pointed a pistol at protesters after he and another man were attacked, according to a Reuters photographer who witnessed the incident. Within a minute or two, about 20 uniformed officers arrived and detained one of the protesters.

Lieutenant Chris Bolton of the Oakland Police Department referred all inquiries about the incident to California Highway Patrol (CHP). "It was a CHP arrest conducted by CHP officers," Bolton wrote on Twitter.

The California Highway Patrol did not immediately respond to multiple requests for comment, including whether one or more of its officers were working undercover.

Two subway stations in downtown Oakland were shut for a time late on Wednesday due to the protest, city officials 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.