The stated goals of the march are to affirm science as a crucial part of a strong democracy, show support for the scientific community, and highlight the value of fact-based inquiry and policy-making.
The Washington event featured speakers and several large teach-in tents on the National Mall where scientists, educators and leaders from a variety of disciplines discussed their work, effective science communication strategies and training in public advocacy.
"Their inclination is misguided and in no one's best interest".
Michael Mann, a climate scientist who regularly clashes with politicians, said: "We didn't choose to be in this battle, but it has come to the point where we have to fight because the stakes are too great". They are fed up with numerous current administration's policies, such as the proposed $5.8 billion cut to the National Institutes of Health and proposed $2.6 billion cut to the Environmental Protection Agency.
"Unfortunately, right now in this country valid, realistic scientific research is threatened", Palmer said.
Marchers held signs with slogans such as, "The oceans are rising and so are we", and, "Denial is not a policy", CBS New York reports.
"That guy over there", the musician and producer said from a stage north of the Washington Monument.
In Raleigh marchers gathered at Shaw University in the morning and walked through downtown to Moore Square, where there was a science fair and rally.
Joni Wright, a neurophysiology graduate student at the University of Florida, cited the Trump administration as a reason why she was in the crowd.
Asked at the White House's roll-out of the spending proposal about the cuts to climate change-related programs, Office of Management and Budget Director Mick Mulvaney said those programs are "a waste of your money".
There was union participation, from nurses to San Francisco's engineer and scientists' union. Coinciding with Earth Day, the March for Science aims to "defend the role science plays in our health, safety, economies, and governments".
Many scientists have been grilled on just how much humans have affected the climate.
"In general, more people are accepting of science", she said.
It wasn't only major cities where scientists and their supporters came out.
Trump complains about measuring him by his first 100 days
Trump also signed two presidential memoranda aimed at the Dodd-Frank financial reform law. His disapproval rating was higher, at 51 percent.
Meike Weltin is a doctorate student at an environmental institute near Berlin.
Demonstrators also turned out in Melbourne, Brisbane, Perth, Adelaide and other cities as well as Wellington and Auckland in New Zealand. One marcher carried an erasable lab-room whiteboard for posting his signs, so he could erase them and update as warranted.
Marches were also held in Durban and Cape Town, South Africa, and in Tokyo.
Marchers said they're anxious about political involvement in science that rejects, for instance, climate change, environmental concerns and the safety of vaccines.
There are cancer survivors and doctors with signs that say "science saves lives", she said, and estimated that 90 percent of the signs are not political. Organizers of the march listed 610 satellite marches around the world, from London to Guam. "And we really need a culture change".
"If we don't support our science, we're going to lose it", Zurawski said in an interview.
Castoldi said it felt good to be around likeminded people.
US President Donald Trump himself passed dozens of protesters on his way to visit wounded soldiers at a military hospital. "Hopefully we reach them in some way to tell them that science is apolitical".
It might have been ignited by Trump, but it's not about Trump.
Andrew Freedman contributed reporting.
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