(McKenna Ewen/The Washington Post)

Thousands of people gathered in the rain Saturday on the soggy grounds of the Washington Monument to turn Earth Day into an homage to science. After four hours of speeches and musical performances, they marched down Constitution Avenue to the foot of Capitol Hill, chanting “Build labs, not walls!” and “Hey, Trump, have you heard, you can’t silence every nerd!”

The March for Science began as a notion batted around online on Reddit after the Women’s March on Washington, which was held Jan. 21, the day after President Trump’s inauguration. The idea snowballed after it was endorsed by numerous mainstream science organizations, which vowed that it would not be a partisan event. It eventually became a global phenomenon, held in more than 600 cities on six continents — and cheered on by scientists on a seventh, Antarctica.

“We are at a critical juncture. Science is under attack,” said Cara Santa Maria, a science communicator who is one of several emcees of the four-hour rally that kicked off at 10 a.m. “The very idea of evidence and logic and reason is being threatened by individuals and interests with the power to do real harm.”

She went on: “We’re gathered here today to fight for science. [The crowd cheered.] We’re gathered to fight for education. [Cheer.] To fight for knowledge. [Cheer.] And to fight for planet Earth.” [Cheer.]

She was followed by the musician Questlove, who said “many people” are refusing to follow scientific facts, and he pointed toward the White House. “That guy over there,” he said in a whisper. He waved, said “Hi,” and made a fast gesture with his middle finger that someone not paying close attention might well have missed.

YouTube star Tyler DeWitt took the stage with another pointed message: Experts need to learn how to explain things in a way regular folks can understand.

“Ditch the jargon!” he said. “Make it understandable. Make people care. Talk to them, not at them. We cannot complain about slashed funding if we can’t tell taxpayers why science matters.”

Cell biologist Lydia Villa-Komaroff, one of the march’s honorary co-chairs, told how she and her colleagues in the 1970s discovered how to make insulin in bacteria, and how that breakthrough was made possible only through basic research funded in the 1950s and 1960s when no one knew if it would lead to anything. “Support for science has been declining for decades. Mr. President, members of the House and Senate, support our future. Invest in science!” she said.

Denis Hayes, co-founder of the first Earth Day in 1970, chose not to dial back his rhetoric, saying the White House “reeks of greed and sleaze and mendacity” and declaring, “America has had 45 presidents, but we have never before had a president who is completely indifferent to the truth. Donald Trump makes Richard Nixon look like Diogenes.”

President Trump issued an Earth Day statement that did not mention the March for Science directly, though seemed to be aware of what was happening within shouting distance of the White House. After pledging to keep the nation’s air and water clean and protect endangered species, the president said:

“Rigorous science is critical to my Administration’s efforts to achieve the twin goals of economic growth and environmental protection. My Administration is committed to advancing scientific research that leads to a better understanding of our environment and of environmental risks. As we do so, we should remember that rigorous science depends not on ideology, but on a spirit of honest inquiry and robust debate.”

At times, the lines to get through the event’s security checkpoints stretched for several blocks. The advanced technologies known as the umbrella and the rain poncho proved useful. The program ran so precisely on schedule, you would think it had been timed with an atomic clock. People danced when Thomas Dolby took the stage to perform his 1982 blockbuster hit “She Blinded Me With Science.”

Some people wore lab coats. Some wore pink, knitted “brain” hats. Sam McCoy, 27, who traveled from North Carolina, carried a homemade sign certain to baffle anyone lacking an understanding of P Values and the null hypothesis. But most of the signs were more straightforward:

●“In peer review we trust.”

●“The oceans are rising, and so are we.”

●“If you’re not part of the solution, you’re part of the precipitate.”

As it happens, the National Math Festival is also in Washington — so there’s an unusual number of people in town who can recite pi to more than five digits.

Some signs jabbed the current occupant of the White House:

●“Hey Trump — Think You Can Stifle Science? Ask Galileo How That Worked Out!”

●“Empirical Data Trumps Imperial Alt-Facts.”

As marchers waited in a glacially moving line for the bag check, and huddled under umbrellas, 60-year-old Cathy Butler implored everyone to join her in a chant.

“Science, not silence! Science, not silence!” she shouted.

A few of the protesters joined in halfheartedly.

Butler sighed.

“I get it! We’re scientists, and we’re all introverted!” she said. “But this is the time that we’re supposed to be loud!”

Butler, a retired engineer and educator from Kennett Square, Pa., said she has been dismayed to see the Trump administration’s efforts to roll back environmental regulations and cut funding for research. Still, she said, she has been trying not to get too political.

“Even people who voted Republican can still get behind clean air for their grandchildren,” she said.

Next to her, 44-year-old Jeffrey C. Jacobs of Herndon, Va., agreed.

“We’re not here for partisanship,” said Jacobs, who runs a science book club. “There are many Republican scientists. Science helps everyone.”

Three federal scientists, approached by a reporter, refused to give their names for fear of repercussions at work.

Another person was dressed as the Muppet “Beaker” and, when interviewed, would say only “Meep.”

Next to Beaker, however, was Erik Molvar, director of the Western Watersheds Project, who had traveled from Wyoming. Molvar is a sage grouse expert who studies the impact of livestock on grouse habitat. Politicians supporting the livestock industry ignore his research into cheatgrass, which is highly flammable and leads to damaging wildfires, he said. “Livestock spread cheatgrass like mosquitoes spread the Zika virus,” he said.

Emily Fink, 28, and Kayla Denson, 29, are biomedical researchers who drove seven hours from Buffalo to attend the march, and they said they fear the Trump administration’s proposed budget cuts will imperil their careers.

“It feels like we’re getting our foot in the door right as the door is closing on us,” Fink said.

Fink brought several copies of her résumé to the march and held up a neon sign that read, “Are you looking for a highly motivated post doc? Ask for CV.” She thought the march might be a good networking opportunity, though so far no one had asked for a résumé.

“I’m a meteoriticist,” said Conel Alexander, 56, as he wrote down his occupation to make sure he spelled it correctly. He studies meteorites and was with a group from the Carnegie Institution for Science. “Most people just think I’m a meteorologist,” he said.

Speaking of meteorology: Favorable weather forecasts took a turn by week’s end, and this turned out to be a soggy Saturday in which periodic showers gave way to heavy late-morning downpours.

But many people present are veterans of protests. Grace Francis, 33, and Ron Frerker, 36, of Washington, said they’ve been to a series of protests since Trump took office. Francis, a former public school teacher who conducts special-education research at George Mason University, said when she heard about the science march she asked her daughters Sage and Eleanor, who are 5 and 3 years old, respectively, if they wanted to go.

“It’s important for them to see that not just our family feels this way,” she said.

The two girls were dressed in koala costumes, and holding signs that read “ DeVos is un-koala-fied” and “everyone deserves a Koala-ity education.”

“What do we feel about schools,” Francis asked Sage.

“Bad,” Sage answered.

“Because everybody deserves to have a . . . ”

“Education.”

“What kind of education?”

“A good one.”

Maryam Zaringhalam and Kelly Fleming, both 28, came with poster board signs they’d made at an event Friday night with the group 500 Women Scientists. Zaringhalam, a molecular biologist, and Fleming, a chemical engineer, had been concerned about the way diversity issues were dealt with by the march organizing committee.

“But I thought, people are going to be taking pictures at the march and this is what I want them to see,” Zaringhalam said. “I want them to see someone who looks like me.”

Zaringhalam is an Iranian American who was in Iran when Trump issued his executive order on immigration. Although she is a U.S. citizen, Zaringhalam worried she would not be allowed back into the country.

At their sign-making event Friday night, a passerby had asked what the women were doing. When they told him, he responded, “You don’t look like scientists.”

“I think he thought he was flirting,” Fleming said, making a face. Zaringhalam came up with the motto for her sign: “This is what a scientist looks like.”

Craig Fryer, 47, marched down Constitution Avenue alongside several of his colleagues from the University of Maryland School of Public Health, all wearing T-shirts that read “Black Scientists Matter.” Fryer, a behavioral scientist who studies substance abuse, carried a sign that proclaimed “Black Scientists Speaking Truth to Power.” He said he and his colleagues are concerned about racial disparities in funding for research.

“We need increased funding, not budget cuts, and we need an equal opportunity to be funded,” Fryer said.

Carol Trosset, 57, an anthropologist at Carleton College, traveled to Washington from Northfield, Minn., having never been to a political rally before. She wore the lab coat — now quite the vintage item — that had belonged to her late mother, who had been a PhD chemist in the 1940s and 1950s.

(Claritza Jimenez/The Washington Post)

“I thought, what should I wear? I’ll wear mom’s lab coat,” Trosset said. Her mother was also a naturalist, collecting data at their home in Cincinnati, recording when birds would arrive and flowers would bloom. Trosset has begun analyzing her mother’s data and sees clear signs of a warming climate.

Brooke Hardesty, 16, waited nervously in front of the science tent. She was looking for the other “Science Teens,” fellow high school students who are slated to speak at the rally. From far-flung cities around the country — Hardesty is from Buckeye, Ariz. — they’d previously communicated only through social media and Skype. On Saturday, Hardesty met her fellow nerds for the first time in person.

“I’ve never been around so many other people who are excited about science,” she said.

The teens, chatting awkwardly, discussed who they’re most excited to see — “Bill Nye!” they said in unison — and talked about how surreal this experience has been.

“I’m kind of surprised they let a bunch of teenagers do this,” said Sam Rosenberg, 17, of Gaithersburg.

“We’re even verified on Twitter!” Hardesty agreed.

The lineup for the rally includes some prominent names in science and environmentalism, including Nye, chief executive of the Planetary Society and an honorary co-chair of the march. But the organizers wanted to erase the stereotype of science as a stodgy enterprise dominated by older white men, and the lineup includes speakers from a broad range of ages, backgrounds and expertise.

They included Taylor Richardson, a 13-year-old aspiring astronaut who raised $17,000 this year to send other girls to see the film “Hidden Figures”; chemist Mary Jo Ondrechen, a member of the Mohawk Nation and chair of the board of the American Indian Science and Engineering Society; and Gallaudet University biologist Caroline Solomon, who is deaf.

“We need more girls in computer science. We need more diversity in computer science,” said speaker Kavya Kopparapu, 16, a student at Thomas Jefferson High School for Science and Technology and founder of the Girls Computing League.

“In my future career, I don’t want to be known as a girl that happens to be a computer scientist. I want to be known as a computer scientist that happens to be a girl,” she told the crowd.

No politicians were given speaking roles, though some reportedly planned to show up for the march.

Some scientists in recent weeks have said they worried the march would politicize the broader scientific enterprise and signal an alignment with left-leaning ideologies. The march’s website offered an answer to that concern: “In the face of an alarming trend toward discrediting scientific consensus and restricting scientific discovery, we might ask instead: can we afford not to speak out in its defense? There is no Planet B.”

Molly Jung, 29, a doctoral candidate at Johns Hopkins, echoed that view: “It’s time for scientists to get out of the ivory towers and get the message out.”

The Washington march may have been the biggest gathering — organizers received a permit for up to 75,000 people — but there were similar events in more than 600 cities on six continents. Seven researchers in Antarctica went on Twitter to express their support for the march. Thousands of people gathered in Sydney, Australia, according to the Sydney Morning Herald. Science advocates went on social media to post bulletins from marches in Austria, England and Malawi, among other places.

Jia Naqvi, Kayla Epstein, Perry Stein, Martine Powers and Taylor Hartz contributed to this report.