A crowd of thousands booed a vision of a world without science: The Environmental Protection Agency’s staff cut by a third, its science by half. Dirty water, polluted air, unchecked climate change.

“As Americans, as New Englanders, as Boston Strong — we care about our natural world!’’ shouted Gina McCarthy, the recently departed head of the US Environmental Protection Agency, as she addressed the March for Science Saturday on Boston Common, which drew enough science supporters to pack the Common along Charles Street from Boylston to Beacon. “Now is the time to speak truth to power!’’

The event — billed as a march but held as a rally because of safety concerns about thousands marching through the streets — was one of more than 600 events held around the world Saturday, which was Earth Day. The events were organized to protest proposed cuts in science funding by President Trump’s administration, as well as the president’s appointment of Cabinet members who are viewed by many as hostile to scientific concepts such as global warming or vaccination.

Organizers and attendees in Boston celebrated the role of scientific inquiry in human life, many carrying signs that listed advances the world would lack without it — antibiotics, the eradication of polio and smallpox, dentistry, and beer among them. But while organizers had stressed that the rally was bipartisan, many attendees voiced frustration, anger, and fear at where Trump’s leadership was leading the country.

“In a sense, Trump is like Pope Urban VIII, who denied to Galileo that Earth goes around the sun,’’ said senior Massachusetts Institute of Technology scientist Richard Petrasso, who studies inertial confinement fusion.“Urban VIII,’’ he said with a laugh. “I’m sure he voted for Trump.’’

Audrey Bianco, a 27-year-old therapist from Acton, carried a sign that read “When govt tries to prevent scientists from informing the public, this is not politics, it’s oppression.’’

She came, she said, because she believed in a science-informed democracy, and because attending the rally was a safe way to release her anger.

“No matter what people say, science is the truth,’’ she said.

Hundreds of children attended the rally, which had dozens of tables set up where they could do their own science experiments, like viewing the C. elegans worm through a microscope and inspecting its mutations, or spraying water on a model of a highway to show how salt runs off into nearby water sources.

“The earth should be safe and no one should be hurting the animals that live on it,’’ said 5-year-old Penelope Bourbeau, who came with her mother and clutched a handmade sign proclaiming “Earth is the best home, science is the best.’’

Nearby, 10-year-old twins Kierra and Andrew Martin offered their own messages.

Kierra, whose sign advised that “kindness is everything,’’ worried that if everyone stopped believing in science, the world would “go into chaos.’’ Andrew feared for the Antarctic. They had come, Kierra said, “just fighting for our rights.’’

In addition to McCarthy, speakers included Dava Newman, a former NASA deputy administrator who stepped down in January; and George Church, a professor at Harvard and MIT who developed methods used for the first genome sequence.

All the speakers argued passionately against Trump’s proposed budget cuts, which would result in major drops in funding to the EPA and other scientific endeavors.

Their message went beyond simple support for science funding, though.

Church encouraged everyone — scientist or otherwise — to participate in the national dialogue on science and to contribute to scientific advancement by volunteering for studies as guinea pigs.

“Science is not optional,’’ he said. “We have big decisions to make, and need everyone engaged in this conversation, not just the elite.’’

Newman, like many other speakers, lamented the lack of women and people of color in the scientific field. She also issued a call to people who were not involved in science to see themselves as part of the movement.

“All of you artists, you visionaries, you paint the picture of how we get to Mars,’’ she said. “You tell the history. We need all of you. Everyone’s in.’’

And speaker Steven Holtzman, president and chief executive officer of Decibel Therapeutics, a hearing company focused on repairing hearing damage, sounded a refrain seen in various iterations on signs and in slogans printed and cheered at the rally.

“Modern scientific inquiry is founded on grounding principles, among which is the belief that the quality of the data — not the economic, political, religious, or physical power of their proponent — should determine their authority,’’ he said.

One of the last speakers was Lindsey Silva, who described how doctors and scientists helped her battle a rare form of lymphoma. That included tracking down a stem-cell donor in France, who was “literally, like finding a needle in a haystack.’’

“It’s truly amazing what the body can endure, and it’s also truly amazing what people in this field put into their careers,’’ said Silva, who earned a master’s degree in nursing after her bout with cancer.

“I hope you fight for science, because the future depends on it.’’

Globe correspondent Adam Sennott contributed to this report. Evan Allen can be reached at evan.allen@globe. com. Follow her on Twitter @evanmallen.