WASHINGTON – 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.

At one point, the crowd linked hands to surround the White House, keeping up pressure on President 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 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 if there was 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.

Groups from Maine were among the protesters.

Environmental activist Andy Burt of Edgecomb said about 100 people – including groups from Unity College, Colby College and the College of the Atlantic – rode overnight on two buses to take part in the protest.

Burt, a consultant on climate change and energy to several Maine organizations, including the Maine Council of Churches, helped organize the buses.

The Sierra Club also played a role in organizing the trip to Washington, Burt said.

She said that she and several other Mainers were among 1,524 people arrested in August in front of the White House during a two-week anti-pipeline sit-in. There were no arrests Sunday.

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.

Bill McKibben, founder of the climate safety grass-roots 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.

Burt touched on similar themes.

“We are the base that worked hard to elect him, and we want him to know that we are holding him to his words and that we are encouraging him to stand tall and make the decision not to go forward with the pipeline,” she said.

MaineToday Media Washington Bureau Chief Jonathan Riskind contributed to this report.