Many - from experienced hands who have been at this for years, to middle school students excited to be at their first big rally - consider climate change the defining issue of their time.
"Twenty-five years from now, nobody is going to look back at our era and say, 'Boy, I wonder how that fiscal cliff thing came out,'" said Bill McKibben, founder of 350.org, an environmental group fighting climate change and one of the sponsors of the rally.
"Everyone is going to look back and say, 'Well, the Arctic melted, and then what did you do?' "
The rally came after a week of climate change developments.
In his State of the Union Address Tuesday, Obama said that "for the sake of our children and our future, we must do more to combat climate change." He added that "if Congress won't act soon to protect future generations, I will."
On Wednesday, as a precursor to Sunday's rally, nearly 50 activists, including Philadelphian Eileen Flanagan, were arrested in an act of civil disobedience outside the White House.
The next day, the Government Accountability Office added the financial liability of climate change to its list of "high risk" areas for the U.S. government, and two senators introduced climate change legislation that would impose a fee on carbon emissions.
The Sierra Club, Clean Air Council, Earth Quaker Action Team, Citizens for Pennsylvania's Future and other groups organized bus transportation from this region.
After adding yet another bus on Saturday - Sierra Club organizer William Kramer said he was getting phone calls and e-mails up to the end - 11 buses with more than 500 people on board headed out from King of Prussia, Quakertown, Devon, West Chester and Philadelphia. Several more left from central New Jersey.
For Jean Mollack, 58, a laid-off worker from Doylestown, it was one more in a series of Washington rallies that began with a Vietnam War protest in 1971. "I think we're ruining the world with our dependence on fossil fuels," she said.
If Mollack was an old hand, Grace DiGiovanni, 12, who goes to Green Street Friends School in Philadelphia, was one of the newbies. She said attending the rally was all about her future. "This is for my generation of kids."
Groups from schools and houses of worship joined the buses. Organizers estimated the crowd at 35,000.
Joy Bergey, 57, a policy director for the environmental group Citizens United for Pennsylvania's Future, brought eight youths from Chestnut Hill United Church, where she's a longtime member.
They included Sarah Noonan-Ngwane, 16, who said environmental issues "should be at the core of what happens over the next four years."
And Monica Guess, 17, who said that if the Keystone pipeline gets built, "it changes our whole future."
For Bergey herself, the rally was the continuation of a battle she began in 1979, when she had her first argument with someone who said climate change wasn't happening.
"I will not stop fighting," she said. "I want there to be a livable planet for all God's creatures."
Nancy Grossman, 53, a pharmacist who lives in Jackson, N.J., was worried about climate change even before she saw the destruction that Superstorm Sandy left along the coast.
"It's one disaster after another," she said. "I don't know what other proof people are looking for."
Albert Accoe, 62, a security consultant from West Philadelphia, said he was attending "for my children and grandchildren."
Liz Robinson, 63, who heads the Energy Coordinating Agency in Philadelphia and attended with her entire family, said, "Everybody should be here . . . it's very profitable to burn oil. Unless all of us stand against climate change, it'll be too late."
Contact Sandy Bauers at 215-854-5147, email@example.com, or follow on Twitter @sbauers. Read her blog, GreenSpace, at www.philly.com/greenspace
Bloomberg News contributed to this report.