The March for Science is expected to hold marches in almost 500 communities around the world Saturday, organizers predict.
Scientists have joined students and research advocates in a series of rallies across the world to push back against what they say are mounting attacks on science.
Organizers of the USA events portrayed the march as political but not partisan, promoting the understanding of science as well as defending it from various attacks, including proposed US government budget cuts under President Donald Trump, such as a 20 per cent slice at the National Institute of Health.
"We have to respect science", he said.
The scientific sentiments spread across the globe as physicists, astronomers, biologists and celebrities gathered for a march in London.
Many marchers carried signs, showing their concern about the earth's welfare.
Those wishing to support the marches can also follow a livestream of the events around the world through the organization's website and social media.
Signs and banners at the Washington DC rally reflected anger, humour, and obscure scientific references, such as a 7-year-old's "No Taxation Without Taxonomy". "Every medical breakthrough, their food, clothing, our cellphones, our computers, all that is science-based", said Pati Vitt, a plant scientist at the Chicago Botanic Garden.
"Because we find ourselves in a time where facts are optional and the Philadelphia March For Science is a big way to show our elected officials and others that we value science and have no intention of starting a new Dark Ages".
"I think the profession of science is under attack, and why is that happening?"
"My administration is committed to keeping our air and water clean, to preserving our forests, lakes, and open spaces, and to protecting endangered species", Trump said. "Science has always been political but we don't want science to be partisan".
Attending the People's Climate March was a life-changing experience.
Aditi Gupta, a research scientist at Rutgers University, in New Jersey, said it was essential for people to pay attention to science and that good governance depended on it. We will march and conduct teach-ins to show that we care about the future of our country and to demonstrate that our work helps ensure a society that's healthy, innovative, environmentally friendly, productive and equitable.
"Really, what's happened is scientists are scared to engage with politics", Tran says.