Many messages carried implicit criticism of President Donald Trump, who withdrew from the global Paris Agreement on climate change.
A placard carried by one demonstrator said "Make America Smart Again", giving an alternative take on Trump's "Make America Great Again" pledge.
"No one wants to be led by the gut feelings of our elected officials," said keynote speaker Sheila Jasanoff, a science and technology professor at Harvard. "Good science depends on good democracy," she said.
David Titley, a retired rear admiral who led the Navy's task force on climate change, said: "Science is what separates facts from fallacies, falsehoods and fanaticism. If we ignore and denigrate science we do so at our own peril."
The rally was one of over 200 carried out around the world on Saturday.