Hundreds of thousands scientists and supporters of science gathered around the world to participate in the March for Science, calling for more investment in science research and education.
Protests are scheduled in more than 600 cities worldwide to mark Earth Day on Saturday due to growing concern about US President Donald Trump's climate change scepticism and cuts to research funding.
The organisers of Earth Day have framed the march as a "celebration" of science to counter a growing disregard for evidence-based knowledge.
"It's important to show this [Trump] administration that we care about facts," said Chris Taylor, 24, who was part of an early crowd of about 15,000 who gathered on the Mall for teach-ins on topics like climate change, water quality, and sustainable food.
The speakers in Washington included Bill Nye, an educator and television personality known as "the Science Guy," and Mona Hanna-Attisha, a pediatrician and public health advocate who first called attention to the high levels of lead in the drinking water of Flint, Michigan.
In Berlin, demonstrators carried signs reading "We love experts - those with evidence" and "Science not Silence" for the march from Humboldt University toward the Brandenburg Gate, led by mayor Michael Mueller and the leaders of the city's universities.
"There are no alternatives to facts," read one large banner, referencing a term used by White House official Kellyanne Conway during a dispute with the media over the size of the crowd at Trump's inauguration.
"Science is necessary. In my opinion, empirical science is the key to progress by the culture and civilisation we have developed," said marcher Hagen Esterberg.
In London, demonstrators marched from the Science Museum to Parliament Square in Westminster, holding colourful placards supporting science.
The march, timed to coincide with Earth Day, comes after moves by Trump to cut funding to the Environmental Protection Agency and the National Institutes of Health.
The White House did not respond to a request for comment on Saturday's marches but Trump released a statement promoting his administration's steps to guard against pollution while minimising limits on industry.
"My administration is reducing unnecessary burdens on American workers and American companies, while being mindful that our actions must also protect the environment," Trump said in a statement.
"Rigorous science is critical to my Administration's efforts to achieve the twin goals of economic growth and environmental protection," he said.
In the past, Trump has said climate change was a hoax that was stifling policies to foster economic growth.