A list of scientific luminaries and their supporters have signed a letter in support of international scientific endeavour on the eve of the Global March for Science on Saturday.

Entertainer Tim Minchin has joined Australian of the Year Alan Mackay-Sim, Nobel Laureate and ANU vice-chancellor Brian Schmidt, University of NSW science dean-elect Emma Johnston and broadcaster Adam Spencer to "call for a world that supports, celebrates and learns from science".

The March for Science will take place in hundreds of cities in more than 50 countries on April 22, Earth Day. The focus will be on Washington DC, where science advocates will rally in response to proposed budget cuts to science announced by US President Donald Trump and what many perceive as his interference in independent climate science research.

Australian of the Year Professor Mackay-Sim is a leading expert in stem cell research at Griffith University. He said: "Scientists must publicly discuss the importance of science in political decision-making and convince politicians of all parties to keep investing in science is as important for our future as spending on health and defence."

The letter is signed by 45 leading scientists and public figures and was organised by Science & Technology Australia, the country's peak body for science representing more than 60,000 scientists and technologists.

Science & Technology Australia chief executive Kylie Walker said: "We are extremely fortunate to have solid support for science and technology in Australia, but with a growing distrust and disregard for science around the world, we think it is time to speak out."

Proposed changes to Australia' 457 visa system have drawn criticism from the scientific and research community, which has said it will weaken the international nature of scientific endeavour in Australia by making it much more difficult to attract post-doctoral scientists to Australia.

University of South Australia deputy vice-chancellor Tanya Monro said: "The news around 457 visas is serious issue for us. At a time when other countries are looking inwards Australia has the opportunity to build on the excellence of our scientific research base to attract the best minds to Australia. "

University of Sydney physicist Michael Biercuk said: "On a strict interpretation [of the proposed changes], we are not able to hire people who are coming out of their PhDs internationally," he said. "We really need to sort out this issue."

However, Professor Biercuk has expressed reservations about the March for Science. He said this week: "I fear that scientists protesting in support of their own disciplines may inadvertently reinforce dangerous perceptions that scientists are part of a detached elite."

This sentiment is shared by a number of scientists, including the Australian Academy of Science. 

The secretary for science policy at the Australian Academy of Science, Les Field, said: "The academy strongly supports US scientists who have established the March for Science.

"Science in Australia is in a different situation and while it is good to show solidarity with our US colleagues it's unclear what the Australian marches will achieve."

In Sydney the march will be addressed by former Liberal leader John Hewson and health advocate Simon Chapman. Professor Chapman this week described the Academy of Science as "spineless" and "naive" for not endorsing the march.

However the academy is relaxed about its staff and fellows participating.

Australian biologist Hugh Possingham will attend the march in Washington DC and is a co-signatory to the open letter.

Professor Possingham said: "As we celebrate Earth Day this year, science and hope are firmly at its core. Tens of thousands of people around the world will take part in the March for Science on Earth Day, calling on our elected officials to continue investing in scientific research that serves as the foundation of a strong, healthy and productive society."