They attempted to prove by sheer numbers what polls say is true: Most Americans favor at least limited abortion rights - even with a president in the White House who opposes abortion.
In the twelve years since abortion rights activists last converged on the Capitol, both sides agree the political landscape has changed, raising the stakes dramatically in this presidential election year.
Celebrities and politicians reminded marchers that the country is one vote away in the Supreme Court from overturning Roe v Wade, and they recalled history for a new generation of women.
"This is what life was before choice," said Whoopi Goldberg, as she held up a coat hanger.
Marchers wanted young women especially to learn from the past.
Carole Mehlman, 68, came from Tampa, Fla., to support a cause that has motivated her to march for 30 years, as long as abortion has been legal.
"I just had to be here to fight for the next generation and the generation after that," she said. "We cannot let them take over our bodies, our health care, our lives."
A much smaller contingent of abortion opponents assembled along a portion of the route to protest what they called a "death march." Among them were women who had had abortions and regretted it; they dressed in black.
Tabitha Warnica, 36, of Phoenix, said she had two abortions when she was young. "We don't have a choice. God is the only one who can decide," she said.
Police used barricades and a heavy presence at that site to keep it from becoming a flashpoint. Both sides yelled at each other as the vanguard of the march reached the counter-demonstration.
"Look at the pictures, look at the pictures," shouted abortion opponents, holding up big posters showing a fetus at eight weeks.
"Lies, lies," marchers shouted back.
Police arrested 16 people from the Christian Defense Coalition for demonstrating without a permit and another anti-abortion protester for throwing ink-filled plastic eggs at rally signs.
Activists now aim to translate this movement into one that turns out more pro-abortion rights voters this November.
Women joined the protest from across the nation and from nearly 60 countries, asserting that damage from Bush's policies is spreading far beyond U.S. shores through measures such as the ban on federal money for family-planning groups that promote or perform abortions abroad.
The rally on the National Mall stretched from the base of the U.S. Capitol about a mile back to the Washington Monument. Authorities no longer give formal crowd estimates, but various police sources informally estimated the throng at between 500,000 and 800,000 strong.
That would exceed the estimated 500,000 who protested for abortion rights in 1992.
Advocates contend that abortion rights are being weakened at the margins through federal and state restrictions and will be at risk of reversal at the core if President Bush is re-elected.
"Know your power and use it," Rep. Nancy Pelosi of California, House Democratic leader, exhorted the masses. "It is your choice, not the politicians."
And feminist Gloria Steinem accused Bush of squandering international good will and taking positions so socially conservative that he seems — according to Steinem — to be in league with the likes of Muslim extremists or the Vatican.
Democratic Sen. Hillary Clinton of New York, referring to the 1973 Supreme Court decision legalizing abortion, said the administration is "filled with people who ... consider Roe v. Wade the worst abomination of constitutional law in our history."
Organizers set up voter registration tables; supporters of John Kerry, the Democratic presidential candidate, handed out stickers. The event was not overtly partisan but denunciations of Bush set the tone from the stage and the ground.
The throngs gathered by the Washington Monument for opening speeches and set off along Pennsylvania Avenue, looping back to the Mall near the Capitol. They moved slowly, bottlenecked by their own numbers.
Celebrities familiar to the abortion-rights movement led the parade, among them Whoopi Goldberg, Kathleen Turner and Cybill Shepherd.
Although Roe v. Wade still anchors abortion rights, some states have imposed waiting periods before abortions, requirements that girls under 18 notify their parents, and other limits that have closed abortion clinics or discouraged doctors from performing abortions.
President Bush has signed a ban on what critics call partial-birth abortion, and the first federal law to endow a fetus with legal rights distinct from the pregnant woman.
Abortion rights supporters say a fragile Supreme Court majority in favor of Roe v. Wade could be lost if Bush is president long enough to fill vacancies that come up in the court. Kerry supports abortion rights.
Kate Michelman, president of NARAL Pro-Choice America, said the march was about more than the right to a safe abortion.
"The march is about the totality of women's lives and the right to make decision about our lives," she said.