Older women and college students alike marched on Sunday seeking to influence US policy on abortion before the 2 November presidential vote.
The massive demonstration was sparked largely by what they see as President George Bush's efforts to chip away at women's right to an abortion.
Organisers, who said this protest was larger than the 750,000 who attended the last abortion rights demonstration in 1992, said they were also calling for medically sound sex education, birth control and better health care for women worldwide.
Police did not release their estimates of how many people attended the demonstration, but trains and busloads full of women were arriving from around the country. Organisers estimated about 1.15 million people turned out, updating their earlier figure of 500,000 to 750,000 supporters.
Waving signs that read "Fire Bush" and "Keep Abortion Legal," the crowd packed onto the National Mall - the grassy esplanade that links the Congress, the White House, and America's most revered monuments and museums.
"All the people are here today not only to march on behalf of women's lives but to take that energy into the election in November," Senator Hillary Clinton told the crowd before the march began.
She urged the crowd to vote for the Democratic presidential candidate, Senator John Kerry.
Opponents of abortion rights also turned out in far smaller rival protests, many carrying pictures of aborted foetuses and denouncing what they see as infanticide.
Organisers said many more were
at this rally than in 1992
"It's murder. It's wrong," said one man from San Diego, California, who declined to give his name. "This country has become an abomination before God. What's it going to take for this country to repent and come back to God."
More than 1200 civic groups worked together to organise the protest, sparked by recent efforts to curtail the reach of a landmark 1973 US Supreme Court ruling that recognized women's right to an abortion.
Activists are particularly concerned by two laws passed in the last six months, one which bars some late-term abortions and another that adds criminal penalties for attacks on pregnant women that injure or kill a foetus.
Women's rights activists from dozens of other countries also joined the protest, citing the ripple effect of US policies overseas - especially the so-called "gag rule" that withholds US aid dollars from groups providing access to or counselling about abortion. They blame the restriction for the deaths of 75,000 women who receive unsafe abortions annually.
Abortion is one of the most volatile issues in US politics, but polls show a majority of Americans support the right for women to choose to terminate a pregnancy.
A Gullup poll released on Friday found 48% of respondents consider themselves "pro-choice," while 45% identify as "pro-life."
Asked if current abortion laws should be made more strict, less strict, or left alone, only 37% wanted stricter laws. More people or 40% wanted the laws left unchanged, while 20% thought they should be more liberal.
However, the poll also indicated abortion ranks low on the issues voters say are key to them before the fast-approaching 2 November US presidential election.
It ranks below taxes, foreign affairs and gun policy, according to the Gallup survey of 1000 people. The poll had a margin of error of three percent.
Sixteen people, mainly anti-abortionists with the Christian Defence Coalition, were arrested at the rally, police said.