The march began at 12:30 p.m. near the Capitol, and demonstrators planned to move to the White House and end up at the Washington Monument, according to the proposed route map
Michele Holmes, from New York's Harlem neighborhood, is one of those activists. She joined about 200 others who climbed into four buses traveling to Washington early Saturday to join in the march.
"Trump is undoing everything Obama did. He doesn't realize climate change impacts everyone. It impacts him," Holmes told CNN. "Change is inevitable, and only we can solve it -- the impact is just changing the way we live."
Temperatures neared 90 degrees Saturday, well above the average high of 71 degrees for April 29, according to Weather.com.
Washington's Fire and Emergency Medical Services said it had received 50 calls as of 4:41 p.m. ET for medical incidents related to the climate march. Four people were taken to hospitals.
Hundreds of sister marches were also planned across the United States and around the world.
Protesters marched through the snow in Denver. Demonstrations were held in Boston, New York, Seattle, Chicago, Amsterdam and London.
Coinciding with Donald Trump's 100th day in office, the protests are taking on the President's environmental policies, which have generally prioritized economic growth over environmental concerns.
During those first 100 days, the Environmental Protection Agency has moved swiftly to roll back
Obama-era regulations on fossil fuels while also facing significant planned budget cuts.
Friday, the EPA removed most climate change information
from its website, saying in a statement that the language was updated to "reflect the approach of new leadership."
The protest continues the trend of heightened liberal activism since Trump's election, best seen in the Women's March
in January and the protests at airports to combat the administration's travel ban
"We've already seen just how effective people power is against this administration: Trumpcare? Withdrawn. Muslim ban? Blocked," the protest group's website says.
"Now Trump's entire fossil fuel agenda is next, and we believe that the path forward is based in the voice of the people -- which is expressed first and foremost through mass protests and mass marches."
'Climate change is real'
On Facebook, about 12,000 people RSVP'd
they would attend the event in Washington. Celebrities such as Leonardo DiCaprio
joined in the protest as well.
DiCaprio marched alongside an indigenous rights group, carrying a sign saying, "Climate change is real."
The protest was full of references to Trump and his frequent trips to his Florida resort Mar-a-Lago, including a Trump-like figure teeing off on the Earth.
Joining the march in Washington, 16-year-old Olivia Springberg tried to make the issue of climate change personal to Trump. She held a sign that warned how rising sea levels
could affect Mar-a-Lago.
Playful tweaks on Trump slogans -- "Make America cool again," for example
-- were prominent, as well as signs showing climate change's impact on animal species.
Nando Rodriguez brought his niece, Eva Luna, from New York to Washington for the march as a learning experience, he said.
"The environment is probably the one issue where race, culture, gender (and) age don't matter," Rodriguez said. "Nothing is in separation except the planet or destroying it."
The People's Climate March even got a shoutout from Hillary Clinton, who wrote on Twitter that it was "great to see (people) take to the streets."
The protest comes a week after the March for Science, in which demonstrators gathered in the capital to protest proposed cuts to scientific studies and to promote facts and research. Signs and speakers at that event similarly criticized climate change skepticism in the Trump administration.