Scientists have taken to the streets to protest over fears that long-standing principles underpinning centuries of progress could be at risk.
Several celebrities joined the March for Science in London as it passed a series of the capital's most celebrated research institutions.
It follows the mushrooming of fake news stories and others that purport to be based on fact but are proved otherwise by the scientific community.
It also comes after the Trump administration faced accusations of attacking efforts to restrict climate change and the science on which it is based by slashing the US's Environmental Protection Agency budget.
The march in London was one of hundreds of similar events taking place around the globe, from Australia to the US, in a mass celebration of scientific endeavour.
Doctor Who star Peter Capaldi and comedian Robin Ince were among those seen mingling with physicists, astronomers and biologists, numbering some 12,000 organisers said.
Concerns that the comments of politicians could threaten to override academic research flared up last summer after former Cabinet minister Michael Gove claimed the public "have had enough of experts".
Organisers of the march said it was crucial to highlight "the vital role that science plays in each of our lives and the need to respect and encourage research that gives us insight into the world".
In the capital, supporters gathered outside the Science Museum before marching to Parliament Square past the headquarters of organisations like the Royal Society - the oldest society for the promotion of science still in existence.
There are also concerns that Brexit will result in an exodus of talent and funding from UK research establishments.
The Commons Science and Technology Committee this month said it was important to ensure scientists and experts can still work in the UK following Brexit.
In March, Nobel Prize winning scientist Sir Paul Nurse said: "I don't think there's anything in Brexit that helps universities, either in teaching or research, but we are where we are...we have to make the best of it."
Bristol, Edinburgh, Manchester and Norwich were also due to host marches at the same time.