The Women's March drew members of Congress, world-famous actresses and countless citizens like Joanne Gascoyne of Albany, New York a 78-year-old retired teacher who traveled to New York City with her daughter and two granddaughters.
"I feel people are afraid to vote for a woman for president," Gascoyne said. "I'm really here for women and to inspire my granddaughters to carry on."
The protesters came out for a range of reasons, including immigration, health care and a general antipathy to Trump. But most said they wanted to show support for women and feared that there will be attacks on women's rights during Trump's presidency.
Marches were also held in cities around the world.
The biggest demonstration took place in Washington, where protesters filled Pennsylvania Avenue, the same street Trump walked down a day before during his inaugural parade. In the evening, the crowd moved toward the White House.
On the mall, filmmaker Michael Moore, feminist icon Gloria Steinem, musician Alicia Keys and other speakers emotionally attacked Trump for his views on immigration, Muslims and women.
"It took this horrific moment of darkness to wake us the f--k up," Madonna told the crowd. "It seems as though we had all slipped into a false sense of comfort, that justice would prevail and that good would win in the end."
Historian Douglas Brinkley told CNN that people will always remember the inauguration and the march as momentous events.
"It's big. It's a huge moment," Brinkley said. "It also reminded me, believe it or not, of V-J Day, Victory Over Japan Day, 1945. Because when victory came people started coming out of apartments and homes ... a lot of extra people in these towns just started coming out."
Marches across America
March organizers said protests took place in more than 600 cities across the world.
Attendance was not always easy to determine. For instance, Washington march organizers said about 500,000 people took part, but authorities did not confirm that number or provide their own estimate.
In Boston, 120,000 to 125,000 people protested, according to a police aerial photo analysis cited by a senior Boston Police official. Police said the crowd there was too big for the march route and could not proceed because "it would be like a snake eating its tail."
U.S. Sen. Elizabeth Warren urged Boston protesters to resist: "We can whimper. We can whine. Or we can fight back! We come here to stand shoulder to shoulder to make clear: We are here! We will not be silent! We will not play dead! We will fight for what we believe in!"
In Los Angeles, more than 100,000 people marched, police said. So many people crammed into the streets that "our march turned into a stand," said Ellen Crafts, who handled public relations for the event.
Demonstrators in New York City marched toward Trump Tower -- the President's main residence when he's not in the White House -- but were blocked at Fifth Avenue.
Police said 20,000 people protested in Houston, 60,000 in Oakland. In Atlanta, 60,000 people marched with US Rep. John Lewis, a major Trump critic.
CNN could not independently confirm any of these crowd estimates.
Overall, the protesters were law-abiding, with police reporting only four arrests in 21 American cities. Nobody was arrested in Washington.
Protests even in Antarctica
"Sister marches" happened outside the United States, too.
People gathered to demonstrate in most major cities around the world, including London; Tel Aviv; Melbourne, Australia; Pristina, Kosovo; Moscow; Berlin and Mexico City -- often in front of US embassies.
In Athens, Greece, protesters included refugees from Elliniko Camp, located in the old Athens airport.
There was even a protest in Antarctica
-- about 30 eco-minded tourists and non-government scientists aboard a ship in international waters hoisted signs saying "Penguins for peace" and "Seals for science," organizers said.
Bigger than inaugural?
The march organizers believe more people came out Saturday for the protests than for Trump's inaugural events on Friday.
Washington protest organizers, who originally sought a permit for a gathering of 200,000, said Saturday that as many as a half million people participated, dwarfing Friday's inaugural crowd.
More than 470,000 people had taken the Metro, the Washington subway system, by 1 p.m., a weekend ridership record. The metro tweeted Saturday there were 275,000 trips taken Saturday by 11 a.m.
By comparison, on Friday, there were 193,000 trips by 11 a.m., according to Metro.
Not everyone agrees. Trump, in his first speech after the Inauguration
, said the media under-counted the size of the crowd for his Friday swearing-in ceremony.
"I get up this morning, I turn on one of the networks, and they show an empty field. I'm like, wait a minute. I made a speech. I looked out, the field was, it looked like a million, million and a half people. They showed a field where there were practically nobody standing there. And they said, Donald Trump did not draw well," the President said.
Trump said the unnamed network reported he drew 250,000 people.
"Now, that's not bad," Trump said. "But it's a lie," adding that the inauguration drew significantly more than 250,000 people.
Later Saturday, White House Press Secretary Sean Spicer called his first press briefing
and questioned the media's reporting on crowd sizes, saying the Trump inauguration was "the largest audience to ever witness an inauguration, period."
But Metro confirms numbers first reported in the Washington Post showing that 570,557 riders took trips between the subway system's opening at 4 a.m. Friday through its closing at midnight.
The numbers are lower than what the system saw in 2009 and 2013 during Obama's inaugurations: 1.1 million trips in 2009 and 782,000 in 2013.
'Strength in numbers'
The protesters, which included many men, hit the streets for different reasons, among them health care, the future of the Affordable Care Act, the environment and income equality.
But Trump's stated attitudes toward women and his comments about judging women by their weight and appearance kept coming up, as did his now infamous remarks about grabbing women by their genitalia
"That's just something that really angers me, and I don't think it has any place in my world," Maeve Kelly, 25 of Lambertville, New Jersey. "It feels like everything we have worked toward, and all that suffragettes worked toward -- so we could vote and be successful -- is threatened now."
Leaders and activists from hundreds of left-leaning groups joined the march, including the NAACP, Planned Parenthood, the American Federation of Teachers, as well as pro-immigrant and pro-environment groups.
Margaret Huang, the executive director of Amnesty International USA, which co-sponsored the Washington march, said the group's members were concerned about talk of a Muslim registry.
"These sorts of rhetorical commitments that (Trump) made on the campaign trail cannot become policy," she said.
The protesters said the size of the crowds made a statement in itself.
"It's important to show strength in numbers," said Jennifer Turney of Weathersfield, Vermont, who traveled to Boston to march with three friends. "We're standing up for everyone's rights."