"Today we have a great many lawmakers -- not just here but around the world -- deliberately ignoring and actively suppressing science," one of the event's speakers, TV host and scientist Bill Nye, told a rain-soaked crowd from a stage.
"Their inclination is misguided and in no one's best interest. Our lives are in every way improved by having clean water, reliable electricity and access to electronic global information."
'That guy over there' in the White House
The march, whose beginnings reflect the viral birth of the Women's March on Washington
, was billed by its organizers as political but nonpartisan.
But many messages were leveled at Trump and his party, which holds majorities in Congress. Scientists have raised alarms over Trump's budget blueprint, which would cut $12.6 billion from the Department of Health and Human Services
, including $5.8 billion from the National Institutes of Health alone
One speaker said the administration "tries but fails to silence scientists." Several contrasted rationality and scientific thought to "alternative facts," a phrase that's attracted popular derision since a White House aide uttered it
With the White House in view, protesters held signs with messages such as "In peer review we trust" and "It's the environment, stupid."
The crowd fended off a steady rain with umbrellas and jackets as event co-host Questlove alluded to Trump.
"That guy over there," the musician and producer said from a stage north of the Washington Monument. "It's been frustrating to watch as certain forces in our society try to squelch science or their refusal to believe in it or propose alternative realities and facts -- alternative facts, whatever that (expletive) is."
Demonstrators marched from the National Mall to Union Square.
Joni Wright, a neurophysiology graduate student at the University of Florida, cited the Trump administration as a reason why she was in the crowd.
"Science is really important, and the current administration is making decisions that are counter to climate change, genetically modified food and vaccinations," Wright, 38, told CNN.
Thousands of people also marched in Boston, Los Angeles, San Francisco and New York. In Chicago, throngs marched from Grant Park to the Field Museum for a science exposition, CNN affiliate WBBM-TV reported
"Science has always strived to remain nonpolitical, nonpartisan -- and we're still striving for that," Liz Homsey, a co-organizer, told the station. "Every single scientist at this event feels that it is much more pro science than anti anything."
Marches start in Australia
Demonstrators in Australia kicked off the day of protest.
In Sydney, marchers carried banners, many homemade, with slogans such as "Science makes sense," "Science-based policy = stuff that works," and "Climate change is real, clean coal is not."
Another placard displayed the message, "Governments: stop ignoring inconvenient science!"
It wasn't only major cities where scientists and their supporters came out.
Rebecca McElroy, an astrophysics doctoral student at the University of Sydney, tweeted video of a "mini march for science" around the dome of the Anglo-Australian Telescope in New South Wales.
Demonstrators also turned out in New Zealand cities, including Wellington, Auckland, Dunedin and Christchurch.
New Zealand Green Party co-leader James Shaw tweeted a popular chant from the marchers: "What do we want? Evidence-based science! When do we want it? After peer review!"
Marches were also held in Durban and Cape Town, South Africa, and in Tokyo.
Scientists and their supporters were urged to turn out in force in London as well as other marches in France, Ireland, Finland, Germany, Portugal and the Netherlands.
David Johnson, 35, a doctoral student in London, said he was marching because he felt climate change was being sidelined by the US administration and that science was being demonized.
Another London marcher, 24-year-old student Rachel Denley Bowers, said science was important and that budget cuts affected research.
Roger Morris, professor of molecular neurobiology at King's College London, said: "These marches are brilliant -- a spontaneous, global response led by young scientists empowered by social media, keenly aware of the global challenges that need to be addressed if their world is to have a civilized, sustainable future.
"Insular populist politics, which have temporarily triumphed in the US and UK, need to be balanced by the broader vision of youth."
Katherine Mathieson, chief executive of the British Science Association, said she hoped the marches would be a catalyst for people to think about the role science plays in their lives and a chance for scientists to demonstrate the public benefit of their work.
"Protecting the government's investment in science, particularly when that includes funding for public engagement, is incredibly important," she said. "Science is not just for scientists, and I believe that all of us, whether we work in a lab or not, should have a voice on its future."
Trump's budget proposal, unveiled in March, outlined $54 billion in cuts
across government programs to make way for an increase in defense spending.
US scientists said they fear such a plan would have a major impact on research and science-based policy as well as undermine the importance of science in society and limit future innovation.
"It might have been ignited by Trump, but it's not about Trump," march honorary co-chair Lydia Villa-Komaroff ahead of the event. "It's about the importance of science in society and continuing the support for the science community in keeping our edge."