Scientists and their supporters across the globe are marching in the thousands on Saturday amid growing anxiety over what many see as a mounting political assault on facts and evidence.

Anchored in Washington, with satellite marches planned in more than 600 cities worldwide, the first-ever 'March for Science' was described by organisers as a rallying call for the importance of science in all aspects of daily life.

"The march has generated a great deal of conversation around whether or not scientists should involve themselves in politics," said a statement on the official website, MarchforScience.com.

"In the face of an alarming trend toward discrediting scientific consensus and restricting scientific discovery, we might ask instead: can we afford not to speak out in its defense?"

Organizers say the march is non-partisan and is not aimed against US President Donald Trump or any politician or party, though the Republican US leader's administration has certainly "catalysed" the movement, according to honorary national co-chair Lydia Villa-Komaroff, a molecular and cellular biologist.

She spoke of a growing "disconnect between what science is and its value to society."

"Fundamental basic science really underlies all of modern life these days. We have taken it for granted," Villa-Komaroff told reporters this week.

Trump has vowed to slash budgets for research at top US agencies, including the National Institutes of Health, NASA, the National Oceanic and Atmospheric Administration, and the Environmental Protection Agency, which could lose a third of its staff if Congress approves the proposal.

He also named as head of the EPA Oklahoma lawyer Scott Pruitt, who claimed last month that carbon dioxide is not the main driver of global warming, a position starkly at odds with the international scientific consensus on the matter.

"In the response to this absurdity lies cause for hope," Paul Hanle, chief executive officer of independent scientist and journalists group Climate Central wrote in an op-ed this week.

"Seeing the assault on fact-based thinking, scientists are energised."