Tens of thousands of protesters gathered in Washington to generate pressure on President Barack Obama to take concrete measures to fight global warming.
Waving banners and signs with slogans like "What will be your climate legacy?" the protesters called on Obama to reject the $7 billion Keystone XL pipeline that would bring oil from Canada to Texas, and order the Environmental Protection Agency (EPA) to set carbon standards for power plants, AFP reported.
"We want to challenge president Obama to be a main actor as opposed to being a puppet of the big oil companies," Canadian-born actress Evangeline Lilly, star of TV's "Lost", told AFP. "It's about telling him his speech did not fall in deaf ears."
The event at the National Mall was organized by local and national environmental groups, including the Sierra Club, under an umbrella group named Forward on Climate.
"It's time for the country to wake up, the US has been dragging its feet too long," said Mimi Brody, one of the protesters.
Among the protesters was Nolan Gould, the 14-year-old star of the TV series "Modern Family."
"I'm very hopeful things will change for kids and we'll start realizing we need to take it even further -- take further steps to save this world, because it's all we have and it's not fair to mess it up," Gould told AFP.
"It's by far, by far, the biggest rally on climate in US history," environmental activist Bill McKibben told the crowd as it assembled at the Washington Monument at the Mall. Organizers said that protesters had arrived aboard buses from 28 states.
Van Jones, a one-time Obama adviser on the environment, was strident in his demand for change.
"President Obama, all the good you've done will be wiped out by floods, by fires, by superstorms if you fail to act now!" he told the crowd, indirectly addressing the US leader.
Celebrities who have signed a petition supporting the protesters include Alec Baldwin, Susan Sarandon, Morgan Freeman, Robert Kennedy Jr and Yoko Ono.
The rally comes after the United States last year endured record high temperatures and lengthy droughts, as well as superstorm Sandy, which devastated the New York-New Jersey coastline.
The protest is "very significant," said Jeff Brown, who traveled from the northeastern state of Massachusetts for the demo. "It's demonstrating a strong environmental movement" which is "not fringe, it's mainstream."
Obama mentioned climate change during his inauguration speech in January, and in Tuesday's State of the Union he vowed to take action "for the sake of our children and our future" if Congress fails to do so.
"I will direct my cabinet to come up with executive actions we can take, now and in the future, to reduce pollution, prepare our communities for the consequences of climate change, and speed the transition to more sustainable sources of energy," Obama said in his speech.
The United States is the world's second largest CO2 emitter after China. Soon after taking office in 2009, Obama presented an ambitious measure aimed at reducing carbon dioxide emissions. But the bill ran into stiff resistance from the Republican opposition.