WASHINGTON - Before war protesters ended their demonstration yesterday, several placed cardboard coffins in front of the offices of northern Virginia defense contractors such as KBR Inc. and Lockheed Martin Corp., as riot police stood by.

"Lockheed Martin, you can't hide, we charge you with genocide!" they chanted as part of a demonstration that began in Washington to mark the sixth anniversary of the invasion of Iraq.

Arlington County, Va., police said that there were 2,500 to 3,000 protesters and that no arrests were made.

Organizers from the ANSWER (Act Now to Stop War and End Racism) Coalition said more than 1,000 groups had sponsored the protest to call for an end to the Iraq war, and estimated that 10,000 people participated.

Carrying signs that read "We need jobs and schools, not war" and "Indict Bush," demonstrators beat drums and played trumpets as they marched from near the Lincoln Memorial past the Pentagon into Virginia.

At a similar protest in San Francisco, tensions grew after four or five dozen activists surrounded a group of riot-equipped police, throwing sticks and water bottles. Police regrouped in riot formation and detained several protesters who pushed and shoved with officers.

Protest leaders shouted from the stage, urging police to leave.

Barriers were quickly erected between police and protesters as an organizer urged calm, and the activists started to disperse.

In Washington, protesters demanded that President Obama immediately withdraw all U.S. troops from Iraq, saying that thousands of Iraqis had died and thousands of American troops had been killed or wounded.

"We think it's especially important for this new administration to feel the pressure from people that we don't want more war," said Obama supporter Pat Halle, 59, of Baltimore.

Antiwar activists said that even though former President George W. Bush was out of power, they were disappointed with what they saw as stalled action from Obama.

"Obama seems to be led somewhat by the bureaucracies. I want him to follow up on his promise to end the war," said Perry Parks, 66, of Rockingham, N.C., who said he served in the Army for nearly 30 years, including in Vietnam.

Obama has said he planned to withdraw roughly 100,000 troops by summer 2010. He has promised to pull out the last U.S. troops by the end of 2011, in accordance with a deal Iraqis signed with Bush.

As of March 13, about 138,000 U.S. troops were in Iraq.

In southern California, hundreds of protesters gathered in Hollywood. Among them were peace advocate Cindy Sheehan, whose son was killed in Iraq; Oscar-winning screenwriter Paul Haggis; and Ron Kovic, a paralyzed Vietnam veteran whose story was chronicled in the book and film Born on the Fourth of July.

Protesters in Los Angeles were expected to follow a rally with a march and then a symbolic "die-in," in which they planned to lie down in a major Hollywood Boulevard intersection to symbolize the soldiers who have died.