OTTAWA - Thousands of anti-pipeline protesters gathered in Washington, DC, Sunday to urge US President Barack Obama to reject the TransCanada Keystone XL Pipeline proposal - a project Prime Minister Stephen Harper has called a "complete no-brainer."

The protesters, who marched to the White House to hammer "dirty" Canadian crude, have urged Obama to quash the plan to push Alberta oil to the Texas Gulf Coast over climate change concerns and other environmental fears.

In his State of the Union address last Wednesday, Obama issued an ultimatum that he would work to slash greenhouse gas emissions and address global warming - a move that has left environmental activists hopeful.

"If Congress won't act soon to protect future generations, I will," Obama said. "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."

A contingent of Canadian protesters, including musician Sarah Harmer, travelled to Washington for the demonstration which rallied under the banner, "Forward on Climate Change."

Obama's administration will ultimately determine the fate of the $7 billion project, which has been staunchly supported by the Canadian government and oil lobby groups including the Canadian Association of Petroleum Producers.

Harper has argued the pipeline's construction will create thousands of jobs, billions in revenue and help address America's "enormous" need for energy.

Environment Minister Peter Kent's office would not comment on Sunday's protest.

-with files from Reuters