Thousands of protesters descended on Washington DC on Sunday demanding Barack Obama shut down the Keystone XL pipeline project to show he is serious about taking action on climate change.

Organisers said a crowd of 35,000, carrying placards in the shape of bright red stop signs, gathered at the Washington monument on a bright but bitterly cold day for the march on the White House.

The event, billed as the largest climate protest in history, was intended as a show of force before Obama renders his decision on the pipeline project in the next few months.

Protesters were bussed in from 30 states and Canadian provinces. Marchers held aloft banners proclaiming "Don't be fossil fools" and "It's time to cut carbon" during the rally.

The DC event was accompanied by smaller one in cities including Austin, during which protesters chanted that the president had "sold out Texas" over the pipeline.

Opponents of the project – environmental activists, indigenous Canadian groups, and landowners alone the pipeline route – have cast the Keystone XL as the defining test of Obama's second term.

They said Obama's entire presidential legacy rests on the Keystone XL, which they framed as a critical test of Obama's environmental credentials and his sweeping promises to use the next four years to protect future generations from climate change.

"Obama holds in his hand a pen and the power to deliver on his promise of hope for our children," said Michael Brune, the director of the Sierra Club. "Today we are asking him to use that pen to reject the Keystone XL tar sands pipeline, and ensure that this dirty, dangerous, export pipeline will never be built."

Others said Obama owed it to his supporters to reject a project, which would open up a vast store of carbon. "I think it's really important for Obama to realise that his base, the people who supported him, do not want this," said Judy Dufficy, a former teacher from Chicago. She said she had volunteered for Obama in Iowa, where he started his run for the White House in 2008.

The president is under even stronger pressure from the oil industry, business groups and the Canadian government to approve the project, which will open new outlets for the vast crude reserves of Alberta's tar sands.