WASHINGTON — The moment he appeared on the stage in a black jacket and red bow tie, the crowd hit near-deafening decibels. A sea of iPhones appeared, everyone stretching and jostling for the best possible photo angle. They cupped their hands to their mouths, screaming his name.

“Greetings, fellow citizens,” Bill Nye said to the thousands huddled beneath umbrellas and hand-lettered signs. “We are marching today to remind people everywhere, our lawmakers especially, of the significance of science for our health and prosperity.”

Near the foot of the stage, a young woman with a bright green pixie cut shouted: “I love you!”

It was a significant moment – for science, for William Sanford Nye, and for the masses who have followed him for decades, from fuzzy TV screens in their middle school classrooms to the grounds of the Washington Monument at Saturday’s March for Science. He is beloved by millennials who came of age watching the ’90s-era PBS series “Bill Nye the Science Guy,” a role that made him an icon: half mad professor, half Mr. Rogers, perpetually clad in a pale blue lab coat and bow tie as he unveiled the science of eroding mountains or orbiting comets with theatrical flourish.

More than 20 years later, the 61-year-old still wears the bow ties, and he still punctuates his speech with impassioned catchphrases. (“It’s not magic, it’s science!” is his new favorite.) But now his disheveled locks and vaguely Vulcan eyebrows are streaked with gray, and his persona has assumed a new edge. He’s become more than the zany educator-entertainer who charmed kids with cartoonish sound effects. He is an activist for science, leading those now-grownups into political battle.

Of all the roles he’s played, this is the one he was preparing for all along.

“I did imagine it could come to this,” Nye said Friday..

By “this,” he meant the legions of scientists, doctors, engineers and concerned members of the public taking to the streets of Washington and more than 600 cities worldwide. Their demonstration was a response to the rise of anti-scientific notions – the anti-vaccination movement and climate change denial in particular – and a retort to the Trump administration, which has proposed deep budget cuts for the Environmental Protection Agency and the National Institutes of Health.

In the weeks before the march, many called this kind of mass protest from the scientific community unprecedented. But Nye was not surprised.

The current “anti-science thing,” he said, had been on the rise for decades. “People were denying pollution in 1970, saying it’s a-OK.”

He took note of the early warning signs as a young man in Seattle, where he got his start in broadcasting with a local sketch-comedy show.

“I realized that kids are the future,” he said.