Thousands of protesters rallied outside the White House on Sunday to press US President Barack Obama to scrap plans for a multi-billion-dollar oil pipeline stretching from Canada to Texas.

"Our mainstream society is extremely destructive and exploitative. This is one of the most egregious examples of over-exploitation," Ken Srdjak, 25, an artist from Ohio, told AFP.

Completing the pipeline would mean "harming indigenous peoples and the environment," he argued.

Washington has launched consultations on the 1,700-mile (2,700-kilometer) Keystone XL pipeline which would run from the tar sands of the Canadian province of Alberta to the Gulf of Mexico in the southern United States.

Many environmentalists fear a potential pipeline accident would spell disaster for aquifers in central US Great Plains states. That could disproportionately endanger rural towns and Native Americans, they say.

Thousands of demonstrators, including Oscar-nominated actor Mark Ruffalo and 1997 Nobel peace laureate Jody Williams, crammed Lafayette Square opposite the White House, as Obama was out playing golf.

Hundreds protested in vibrant orange vests reading Stop Keystone XL, while others waved signs with slogans such as "We believe in a better way -- if it doesn't involve tar sands" and "Pipeline to the Apocalypse."

Dozens more, most in their 20s, danced to pop music in front of a soundstage waving their protest banners less than a block from the White House.

Lauren Glapa, 19, rode a bus overnight from Indiana University for her first protest with dozens of classmates to make their voices heard.

"So many people don't know anything about this issue. Obviously Obama does, and I don't know what he is going to do. But if we increase awareness, maybe he will do the right thing," she said, readying for the long ride home.

The Keystone XL pipeline proposed by TransCanada would begin in Alberta in western Canada and pass through the US states of Montana, South Dakota, Nebraska, Kansas and Oklahoma before ending up at refineries in Texas.

A number of environmental and citizen groups are fighting the pipeline because exploiting the unconventional oil sands of Alberta requires energy that produces a large volume of greenhouse gases.

Concerns about potential for an environmental disaster seem to be heightened on the heels of last year's devastating BP oil spill in the Gulf of Mexico.

The US State Department is handling public consultations as the pipeline would run across the border with Canada.

But it said Wednesday it might not decide whether to issue a permit for the proposed pipeline by the end of 2011 as planned.

State Department spokeswoman Victoria Nuland said the "first priority" is to ensure the pipeline's potential environmental impact is carefully studied, not to meet the year-end goal set by Secretary of State Hillary Clinton.