A counterprotest was staged, too, on a day of dueling signs and sentiments such as "Illegal Combat" and "Peace Through Strength," and songs like "The Battle Hymn of the Republic" and "War (What's It Good For?)."
Thousands crossed the Potomac River from the Lincoln Memorial to rally loudly but peacefully near the Pentagon. "We're here in the shadow of the war machine," said anti-war activist Cindy Sheehan. "It's like being in the shadow of the death star. They take their death and destruction and they export it around the world. We need to shut it down."
Smaller protests were held in other U.S. cities, stretching to Tuesday's four-year anniversary of the Iraq invasion.
In Los Angeles, Vietnam veteran Ed Ellis, 59, hoped the demonstrations would be the "tipping point" against a war that has killed more than 3,200 U.S. troops and engulfed Iraq in a deadly cycle of violence.
"It's all moving in our direction, it's happening," he predicted at the Hollywood rally. "The administration, their get-out-of-jail- free card, they don't get one anymore."
Other protests - and counter- demonstrations - were held in San Francisco, San Diego and Hartford, Conn., where more than 1,000 rallied at the Old State House.
Overseas, tens of thousands marched in Madrid as Spaniards called not only for the United States to get out of Iraq but to close the prison for terrorist suspects at Guantanamo Bay in Cuba. Smaller protests were staged in Greece and Turkey.
Speakers at the Pentagon rally criticized the Bush administration at every turn but blamed congressional Democrats, too, for refusing to cut off money for the war.
"This is a bipartisan war," New York City labor activist Michael Letwin told the crowd. "The Democratic party cannot be trusted to end it."
Five people were arrested after the demonstration when they walked onto a bridge that had been closed off to accommodate the protest and then refused orders to leave so police could reopen it to traffic, Pentagon police spokeswoman Cheryl Irwin said. They were cited and released, she said.
President Bush was at Camp David in Maryland for the weekend. Spokesman Blair Jones said of the protests: "Our Constitution guarantees the right to peacefully express one's views. The men and women in our military are fighting to bring the people of Iraq the same rights and freedoms."
People traveled from afar in stormy weather to join the march.
"Too many people have died and it doesn't solve anything," said Ann Bonner, who drove through snow with her husband, Tom O'Grady, and two children, 13 and 10, from Athens, Ohio. "I feel bad carrying out my daily activities while people are suffering, Americans and Iraqis."
Police on horseback and foot separated the two groups of demonstrators, who shouted at each other from opposite sides of Constitution Avenue in view of the Lincoln Memorial before the anti-war group marched. Barriers also kept them apart.
But war protester Susanne Shine of Boone, N.C., found herself in a crowd of counterdemonstrators, and came out in tears, with her sign in shreds. "They ripped up my peace sign," she said, after police escorted her, her husband and two adult daughters from the group. "It was really pretty scary for me." Protesters walked in a blustery, cold wind across the Potomac River with motorcycles clearing their way and police boats and helicopters watching. Police no longer give official estimates but said privately that perhaps 10,000 to 20,000 anti-war demonstrators marched, with a smaller but still sizable number of counterprotesters also out in force. An hour into the three-hour Pentagon rally, with the temperature near freezing, protesters had peeled away to a point where fewer than 1,000 were left. Protesters met at the starting point of the Oct. 21, 1967, march on the Pentagon, which began peacefully but turned ugly in clashes between authorities and more radical elements of the estimated crowd of 50,000 on the plaza in front of the Defense Department's headquarters. More than 600 were arrested that day. That protest has lived on in the popular imagination because of the crowd's attempts to lift the Pentagon off the ground with their chants; they fell short of their fanciful goal. Veterans lined up at the Vietnam Veterans Memorial and waved U.S, POW-MIA and military-unit flags. Not all were committed to the U.S. course in Iraq, however. "I'm not sure I'm in support of the war," said William "Skip" Publicover of Charleston, S.C., who was a swift boat gunner in Vietnam and lost two friends whose names are etched on the memorial's wall. "I learned in Vietnam that it's difficult if not impossible to win the hearts and minds of the people." But Larry Stimeling, 57, a Vietnam veteran from Morton, Ill., said the loss of public support for the Iraq war mirrors what happened in Vietnam and leaves troops without the backing they need. "We didn't lose the war in Vietnam, we lost it right here on this same ground," he said, pointing to the grass on the National Mall. "It's the same thing now." In Sacramento, Calif., nearly 200 veterans and parents of troops gathered on the steps of the state Capitol to rally in support of U.S. troops in Iraq. "This is not a war that can be fought under a white dome in Washington, D.C.," said Kevin Graves, whose son died in Iraq. "If politicians can't support the troops, they should go fight instead." Opening weekend events, more than 200 were arrested in a demonstration late Friday in front of the White House and charged with disobeying a lawful order or crossing a police line. (AP) Associated Press writers Ann Sanner and Cal Woodward contributed to this report. AP-CS-03-17-07 1937EDT