Thousands of protesters have gathered at Washington's National Mall calling on US President Barack Obama to reject the controversial Keystone XL oil pipeline proposal and honour his inaugural pledge to act on climate change.

Organisers of the "Forward on Climate" event estimated that 35,000 people from 30 states turned out in cold, blustery conditions on Sunday for what they said was the biggest climate rally in US history.

Police did not verify the crowd size.

Protesters also marched around the nearby White House, chanting "Keystone pipeline? Shut it down".

The event came days after a bipartisan group of US senators made the latest call for Obama to approve the $5.3bn pipeline, seen by many as an engine for job growth and another step towards energy independence.

A new poll by Harris Interactive showed 69 percent of respondents said they support construction of the pipeline, with only 17 percent saying they oppose it.

'Carbon bomb'

One of Sunday's main organisers, climate activist Bill McKibben, said that approving the pipeline, which would transport crude oil from the oil sands of northern Alberta to refineries and ports in Texas, would be akin to lighting a "carbon bomb" that could cause irreparable harm to the climate.

"There is nothing else you can do if you let that pipeline go through. It doesn't matter what you do on smog rules and automobile rules - you've already given the whole game away." - Van Jones, Rebuild the Dream



"For 25 years our government has basically ignored the climate crisis: now people in large numbers are finally demanding they get to work," said McKibben, founder of the environmental group 350.org.

The proposed TransCanada Corp project has been pending for more than four years. A revised route through Nebraska, which would avoid crossing sensitive ecological zones and aquifers, was approved by that state's governor last month.

Backers of Keystone, which would transport 830,000 barrels of oil per day, say it would provide thousands of jobs in the US and increase North American energy security.

Van Jones, Obama's former green jobs adviser, said if the president approved the pipeline just weeks after pledging to act on climate change, it would overshadow other actions Obama takes to reduce pollution.

"There is nothing else you can do if you let that pipeline go through. It doesn't matter what you do on smog rules and automobile rules - you've already given the whole game way," said Jones, who is president of Rebuild the Dream, a non-government organisation.

Democratic Senator Sheldon Whitehouse of Rhode Island, the lone member of Congress to speak at the rally, told Reuters news agency Obama risked creating a "credibility gap" if he approved the pipeline.

"He would have to roll out a very complete and very strong package to offset something that on its own is described by government scientist as 'game-over' on climate," he said.

Still, some of Obama's core constituents favour the pipeline, including the labour union AFL-CIO's building and construction unit, which sees the potential for job creation for its members, and certain Democratic legislators.

In January, nine Democratic senators joined 44 Republicans in urging the president to approve Keystone XL.