Organizers said the event, planned for hundreds of cities in the US and around the world, would remain non-partisan, though some participants view the march as a way to challenge cuts proposed by the Trump administration that target science programs.
The marches also come at a time when Americans are increasingly pessimistic about the future of the planet.
PM Modi said he hopes this year's theme of "Environmental and Climate Literacy" helps create awareness on protecting nature and its resources.
Here are the highlights from marches and Earth Day events around the globe.
Signs and banners readied for the Washington rally reflected anger, humour and scientific references, such as a seven-year-old's "No Taxation Without Taxonomy".
Organisers were aiming for marches to be a positive celebration of science, rather than an attack on the policies of Trump.
March for Science is the latest in a series of national demonstrations that have been staged since Trump's inauguration almost 100 days ago.
The sign which nine-year-old Sam Klimas held was handmade and personal: "Science saved my life". He says studies have shown good nutrition can save as much as $14,000 a year in medical costs for those fighting illnesses such as cancer and HIV. His mother, grandmother and brother traveled with him from Parkersburg, West Virginia.
"I think climate change is the big issue that's concerning, and that the current administration seems to completely disavow", said Chip Delmar, a Moorestown aerospace engineer, who was sporting a university of Pennsylvania hat during the march.
NPR spoke to some of the participants about why they chose to attend the March for Science.
Moment of truth has arrived for Spurs, says Chelsea's Conte
We were competing with among the top teams in Europe. "When you play football you want to play every game but I think he made a great choice".
At a time when the Earth has marked three consecutive years of record-breaking heat, and ice is melting at an unprecedented rate at the poles, risking massive sea level rise in the decades ahead, some marchers said it was more important than ever for scientists to communicate and work toward solutions to curb fossil fuel emissions.
Thousands of participants walked down Broadway to midtown Manhattan.
After the speakers on the main stage conclude the program, the crowd will march up Constitution Avenue toward the Capitol reflecting pool, ending up and dispersing in front of the Capitol on 3rd Street NW.
When progressives are ready to admit that they sometimes cherry-pick the science they like and disregard the science that confounds their worldview, they will have taken a key first step toward the scientific method.
Gibbs pointed to a 2012 "Death of Evidence" rally at the same location - during which government scientists and their supporters criticized former prime minister Stephen Harper's scientific funding cuts and policies - as an important example that scientific activism is important and works. "We can and must protect our environment without harming America's working families".
Several thousand people participated in a march from the one of the Berlin's universities to the Brandenburg Gate landmark.
Naughton said that in order to promote science in policy making, she believes more scientists need to run for office, and more voters need to pressure their political representatives.
Some March for Science speakers invoked patriotism and love of country as their motivation. We need their voices, speaking accurately and clearly about the challenges we face as a society, from public health to climate change.
In London, physicists, astronomers, biologists and celebrities gathered for a march past the city's most celebrated research institutions.
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