Thousands of scientists are protesting in hundreds of cities around the world against what they see as a global political assault on facts.
The first-ever March for Science, which has been timed to coincide with Earth Day, is aimed at promoting action to protect the environment.
Organisers say it is also a celebration of science and a call to support and safeguard the scientific community.
The main event is due to take place later on Saturday in Washington DC.
The event’s promoters said the march in the US capital was not aimed against President Donald Trump, while adding that his administration had “catalysed” the movement.
From climate change and pollution to medicine, men and women who support science have been motivated by the coverage of the recent Women’s March and are mobilising to make their concerns heard.
Organisers of the March for Science Vienna, in Austria, said on the group’s Facebook page that it was encouraging people to turn out to join a movement that began shortly after Mr Trump entered the White House.
Mr Trump has previously called climate change a hoax and his views have raised concerns among the scientific community that the public are beginning to doubt the facts provided as scientific evidence.
In London, scientists and science enthusiasts marched from the Science Museum to Parliament Square.
Many are protesting against what they see as the “alarming trend” among politicians for discrediting their research.
The aim of the March for Science is to bring scientists and their research closer to the general public.
Organisers have a view that it can be challenging for scientists to communicate with the public and are even encouraging scientists to become politicians so that their voices can be effectively heard.