WASHINGTON (AP) - Thousands of protesters gathered in Lafayette Square across from the White House on Sunday to oppose a plan for a transnational oil pipeline they fear could harm the environment.

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At one point, the crowd linked hands to surround the White House, keeping up pressure on President Barack Obama as his administration decides whether to approve the massive Keystone XL project.

Demonstrators chanted "yes we can, stop the pipeline," while other protesters carried a plastic tube simulating the pipeline that would run 1,700 miles through six states.

The protest drew support from actor Mark Ruffalo, Presidential Medal of Freedom winner John Adams and NASA scientist James Hansen, each of whom spoke to the crowd.

The proposed pipeline by developer TransCanada would carry oil derived from tar sands in Alberta, Canada, to refineries in Houston and Port Arthur, Texas.

Opponents say it would bring "dirty oil" that requires huge amounts of energy to extract and could cause an ecological disaster in case of a spill.

They are calling on Obama to block the $7 billion project, which is currently being reviewed by the State Department.

Obama missed most of the protest while he played golf at Fort Belvoir in Virginia during the afternoon.

Dan Quigley, a freshman at St. Michael's College in Colchester, Vermont, traveled by bus with about 40 students to attend the protest. The 19-year-old said the pipeline could have an adverse effect on greenhouse gases and pose a hazard to water supplies.

"It's putting a lot of time and effort into something that's pulling us into a wrong direction," he said. "If we are going to do anything this large scale it has to be something that's proactive for helping the environment."

TransCanada spokesman James Millar said the pipeline would help reduce American dependence on oil from the Middle East and Venezuela while creating thousands of new construction and manufacturing jobs.

"Killing Keystone just leads to more of the same - hundreds of oil tankers shipping millions of barrels of higher priced oil across our oceans to American shores," Millar said in a statement.

Bill McKibben, founder of the climate safety grassroots movement 350.org, said demonstrators hoped Obama would live up to the image that helped him win election in 2008.

"He's completely capable of doing the right thing," McKibben said.