Former US attorney general Ramsey Clark called for Bush's impeachment, while Cindy Sheehan, who lost a son in Iraq, demanded a US withdrawal.

"I marched in 1967 here," Maureen Dooley, 59, said outside the Pentagon, site of Vietnam war protests, but results were not immediate: "It took seven years to end the war."

War opponents trickled into Washington for the rally organized by the peace group ANSWER as Vietnam war veterans wearing black leather jackets gathered nearby for a counter-demonstration.

Some war supporters confronted the peace activists, tearing up and spitting on anti-war signs while chanting: "USA! USA!"

Washington police do not give crowd estimates, but an AFP correspondent said tens of thousands of people could be seen at the march.

War opponents have organized a series of protests against the conflict that started March 20, 2003.

Demonstrations against the war also took place in several European cities, with turnout ranging from some 6,000 in Istanbul to several hundred in Copenhagen, Prague, Athens and Thessaloniki in Greece.

Alan Pugh, a 27-year-old computer student from Ohio, said he hoped the Washington protest would have the same impact as the mass demonstrations denouncing the Vietnam war decades ago.

"This is the 40th anniversary of the Vietnam protest that changed the direction and we hope we can do the same thing today," he said.

Late Friday, about 100 people were arrested as they held a vigil on a sidewalk in front of the White House and ignored police orders to disperse in a protest organized by Christian Peace Witness for Iraq.

Other demonstrators were planned in Los Angeles and San Francisco.

United for Peace and Justice, which describes itself as the largest anti-war coalition in the United States, said it expected thousands of people to turn up at a protest in New York on Sunday.

"The national anti-war movement is planning a unified surge of protest actions calling on Congress to end the occupation and for the immediate withdrawal of US troops," the group said in a statement.

The leftist group MoveOn.org was also organizing candlelight vigils for Monday in Washington and across the country, spokesman Steve Hoffman said.

The war has grown increasingly unpopular, with recent polls showing that a majority of Americans now say the invasion was a mistake and want the US government to set a timetable for the withdrawal of US troops from Iraq.

Peace activists want the US Congress, secured by the Democrats in November elections that were marked by voter anger at the war, to push hard for the withdrawal of US troops from Iraq.

But Democrats have so far failed to pass legislation that would compel Bush to change course in Iraq.

Democrats failed to pass in the Senate a plan to withdraw US troops by March 2008, although a measure calling for a pullout by September 2008 passed a key panel in the House of Representatives on Thursday.