The protesters included national figures - civil rights leader Jesse Jackson, actress Susan Sarandon and the prominent anti-war activist Cindy Sheehan.
The mass rally was organised by a broad coalition of groups representing veterans, trade unions, military families, environmentalists and civil rights activists.
"This is what democracy looks like," said chief organiser Leslie Cagan, national coordinator of the anti-war lobby United for Peace and Justice.
"We are here to send a message to the White House and the Congress that we must end this war now, bring our troops home and also end the attack on our civil rights at home."
While the main focus of the march was Iraq, Cagan said the rally was also aimed at protesting any plans for military intervention in Iran.