WASHINGTON (Reuters) - Thousands of protesters gathered on the Washington's National Mall on Sunday calling on U.S. President Barack Obama to reject the controversial Keystone XL oil pipeline proposal and honor his inaugural pledge to act on climate change.

A demonstrator holds up a sign during a march past the White House to protest against the Keystone XL pipeline in Washington, February 17, 2013. The TransCanada Corp pipeline would link the oil sands of northern Alberta, the world's third largest crude resource, to refineries and ports in Texas. Environmentalists say approval of the pipeline will encourage more development in the oil sands, where extraction is carbon intensive, leading to greater greenhouse gas emissions. REUTERS/Richard Clement

Organizers of the "Forward on Climate" event estimated that 35,000 people from 30 states turned out in cold, blustery conditions for what they said was the biggest climate rally in U.S. history. Police did not verify the crowd size.

Protesters also marched around the nearby White House, chanting "Keystone pipeline? Shut it down." Among the celebrities on hand were actresses Rosario Dawson and Evangeline Lilly, and hedge fund manager and environmentalist Tom Steyer.

The event came days after a bipartisan group of U.S. senators made the latest call for Obama to approve the $5.3 billion pipeline, seen by many as an engine for job growth and another step toward 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.

One of Sunday's main organizers, 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.

"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.

Other major organizing groups on Sunday included the Sierra Club and the Hip-Hop Caucus.

The proposed TransCanada Corp project has been pending for 4-1/2 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 United States and increase North American energy security.

Environmentalists oppose the pipeline because the oil sands extraction process is carbon intensive, and say the oil extracted is dirtier than traditional crude oil.

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 organization.

Democratic Senator Sheldon Whitehouse of Rhode Island, the lone member of Congress to speak at the rally, told Reuters 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 lawmakers.

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

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