Organizers say they never expected this many to participate.
PHOTOS: Women's March demonstrations in Houston
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"This crowd is amazing. We expected a few hundred people when we planned this, and it has grown and grown and grown. We had ten women working 24/7 to get this done, and it came through," said march organizer Star Massing.
While one voice can make a difference, they say together their statement is even more powers.
The marchers said they want to send a message to the new Trump administration. They support health care, women's rights, education and diversity, and are concerned his administration may not share those values.
"Trump is the president, like it or not. I feel we need to hold him accountable. He's done a lot of things, I think my sign says it all. He talks about oh we're all coming together but he's got to do something to make us feel like we're all coming together," said one marcher who asked not to be identified.
Lauren Price held her sign proudly and added, "Men, women, children, all ages, all genders, all nationalities. It was a great day."
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Meanwhile, turnout in Washington, D.C. was so big that the original march route alongside the National Mall was packed, and instead of trekking en masse to the Ellipse by the White House as planned, the protesters were told to make their way there on their own by way of other streets. In Chicago, organizers canceled the march portion of their event for safety reasons after the overflow crowd reached an estimated 150,000.
The women brandished signs with slogans such as "Women won't back down" and "Less fear more love" and decried Trump's stand on such issues as abortion, health care, gay rights, diversity and climate change. Their message reverberated at demonstrations around the globe, from New York, Philadelphia, Chicago and Los Angeles to Paris, Berlin, London, Prague, Sydney and beyond.
VIDEO: Demonstrators gather in downtown Houston
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"I feel very optimistic even though it's a miserable moment," said Madeline Schwartzman of New York City, who brought her twin 13-year-old daughters to the Washington rally. "I feel power."
Officials said the crowd in Washington could be more than half a million people, more than double expectations. The event appeared to have attracted more people than Trump's inauguration on Friday, based on figures from transportation officials.
More than 600 "sister marches" were planned around the world. Organizers estimated 3 million people would march worldwide.
The rallies were a peaceful counterpoint to the window-smashing unrest that unfolded on Friday when self-described anarchists tried to disrupt the inauguration. Police used pepper spray and stun grenades against demonstrators. More than 200 people were arrested.
Marlita Gogan, who came to Washington from Houston for the inauguration, said police advised her family not to wear their "Make America Great Again Hats" as they walked through crowds of protesters while playing tourist on Saturday.
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"I think it's very oppressive," she said of the march atmosphere. "They can have their day, but I don't get it."
On the streets, feminist leader Gloria Steinem described the worldwide mobilization as "the upside of the downside: This is an outpouring of energy and democracy like I have never seen in my very long life."
"Sometimes we must put our bodies where our beliefs are," she told the crowd, labeling Trump an "impossible president."
Hillary Clinton, who lost to Trump, took to Twitter to thank the participants for "standing, speaking and marching for our values."
The marches displayed a level of enthusiasm that Clinton herself was largely unable to generate during her campaign against Trump, when she won the popular vote but he outdistanced her in the electoral vote.
At rallies around the world, many participants wore hand-knit "pussyhats" - a message of female empowerment aimed squarely at Trump's crude boast about grabbing women's genitals.
They "ain't for grabbing," actress Ashley Judd told the Washington crowd.
The marches were a magnet for A-list celebrities, unlike Trump's inauguration, which had a deficit of top performers.
Alicia Keys sang "Girl on Fire" for the Washington crowd. Madonna addressed the gathering. Cher, also in the nation's capital, said Trump's ascendance has people "more frightened maybe than they're ever been."
In Park City, Utah, it was Charlize Theron leading demonstrators in a chant of "Love, not hate, makes America great." In New York, actresses Helen Mirren and Cynthia Nixon and Whoopi Goldberg joined a crowd of protesters marching to Trump's local home.
Tens of thousands of protesters squeezed into London's Trafalgar Square. In Paris, thousands rallied in the Eiffel Tower neighborhood in a joyful atmosphere, singing and carrying posters reading "We have our eyes on you Mr. Trump" and "With our sisters in Washington." Hundreds gathered in Prague's Wenceslas Square in freezing weather, mockingly waving portraits of Trump and Russia's Vladimir Putin.
"We are worried about the way some politicians talk, especially during the American elections," said organizer Johanna Nejedlova.
In Sydney, thousands of Australians gathered in solidarity in Hyde Park. One organizer said hatred, bigotry and racism are not only America's problems.
The Associated Press contributed to this report.