WASHINGTON — Thousands of scientists and their supporters, feeling increasingly threatened by the policies of President Trump, gathered Saturday in Washington under rainy skies for what they called the March for Science, abandoning a tradition of keeping the sciences out of politics and calling on the public to stand up for scientific enterprise.

As the marchers trekked shoulder-to-shoulder toward the Capitol, the street echoed with their calls: “Save the E.P.A.” and “Save the N.I.H.” as well as their chants celebrating science, “Who run the world? Nerds,” and “If you like beer, thank yeast and scientists!” Some carried signs that showed rising oceans and polar bears in peril and faces of famous scientists like Mae Jemison, Rosalind Franklin and Marie Curie, and others touted a checklist of the diseases Americans no longer get thanks to vaccines.

Although drizzle may have washed away the words on some signs, they aimed to deliver the message that science needs the public’s support.

“Science is a very human thing,” said Ashlea Morgan, a doctoral student in neurobiology at Columbia University. “The march is allowing the public to know that this is what science is, and it’s letting our legislators know that science is vitally important.”