Hundreds of thousands of activists from the US and some 60 other countries have rallied in Washington in support of abortion rights.
Abortion rights activists filled the Washington Mall
Organisers of the March for Women's Lives say women's reproductive rights have never been so threatened, nor has their health faced such peril.
Pro-life activists, who equate abortion with murder, also rallied in Washington but were vastly outnumbered.
Abortion is one of the most divisive and emotive issues in US politics.
The event was the first large-scale abortion rights demonstration in Washington since 1992, the New York Times reports.
Bush under fire
Reports on the size of the crowd vary, but police sources informally estimated it at between 500,000 and 800,000 according to AP, while organisers put it at 1.15 million, AFP reports.
The march organisers included the American Civil Liberties Union and the Planned Parenthood Federation,
"The time is right for a public demonstration of historic size in support of reproductive freedom and justice for all women," they say on their website.
Pro-choice campaigners believe legal abortion is under creeping assault from Republicans who are trying to dismantle its legal foundation bit by bit.
President George W Bush's administration was filled with people who view the Supreme Court ruling that legalised abortion as "the worst abomination of constitutional law", US Senator Hillary Clinton said.
She told protesters the Bush administration was going further - "turning the clock back... on so many issues that are important to women".
"We didn't have to march for 12 long years because we had a government that respected the rights of women," she said, urging the protesters to vote for Mr Bush's rival John Kerry.
Foreign activists said they were attending not only to show solidarity with their US counterparts but to highlight the impact of American policies on women abroad, especially in developing countries.
They cited a decision by President George W Bush in 2001 to enforce what is known as the global gag rule. This prohibits non-governmental groups that perform abortions or offer counselling on abortion from receiving US government funding.
"Since the global gag rule is affecting family planning worldwide, this is also our cause," Catherina Hinz of the German Foundation for World Population told AP.
The US authorities insist that the amount of funding for foreign family planning services remains the same and that the level of service provided has not changed at all.