Thousands of demonstrators protested the proposed Keystone XL at the National Mall on Sunday.

"Hey hey, ho ho, Keystone pipeline's got to go!" protestors chanted at one point.

"I'm ashamed of what my country is doing. I'm ashamed we're knocking on your door with dirty oil. I want to stand up here as a Canadian and say I'm sorry to the workers in Canada and the workers in America who have to go home and look their kids in the eye and know that they are damaging their future," said Evangeline Lilly, a Canadian actor.

The pipeline, if approved, would transport tar sands -- which can be processed and refined into oil -- from Canada through the U.S. Opponents believe the creation of the pipeline will increase pollution and worsen climate change. The Sierra Club, one of the largest grassroots environmental organizations in the U.S., claims on its website that tar sands extraction "leaves behind giant toxic lakes that are linked to abnormally high rates of cancer in neighboring communities and are large enough to be seen from space."

Organizers claim about 40,000 people showed up, reported NBC News, though the news outlet could not verify that number.

A member of the Ponca Nation of Oklahoma called it "environmental genocide."

"We're here to be in solidarity with all of us who understand that we have a very slim opportunity to make human life continue to exist," she said.

Actor Rosario Dawson said, "We'll, I'd like to have a definitive 'no' on the XL pipeline. That doesn't mean that our work is going to stop because there is other pipelines that are already in the works, and in conversation. But, I think we need a win on our side to really show that corporations are not people."

The Sierra Club, a major environmental organization in the U.S., made news recently when its executive director Michael Brune was arrested at a Keystone XL protest a few days -- the first time that a high-level Sierra Club employee has been arrested since it was established 120 years ago.

NBC News reported a decision from Obama is believed to be coming soon.

Watch the video, via MinWashingtonNews on YouTube, below.