Thousands of protesters opposed to a controversial pipeline project surrounded the White House on Sunday.
Canadian company TransCanada is seeking permission to build the 1,600-mile (2,700km) Keystone XL pipeline from Alberta to the Gulf coast in Texas.
Environmentalists are opposed to the project because of the method used for extracting petroleum from Alberta's oil sands.
They are also concerned by the risk of pollution on the pipeline route.
The proposed pipeline would pass south from Alberta through the US states of Montana, South Dakota, Nebraska, Kansas and Oklahoma before ending up at refineries in Texas.
Environmental groups say that extracting oil from the sands would generate huge greenhouse gas emissions, and that any accident on the route could be disastrous.
The protesters formed a human chain around the White House, with some carrying an inflatable replica of a pipeline on their shoulders.
Bill McKibben, one of the protest organisers, described the scene as either "a big O-shaped hug" or "a symbolic house arrest," according to the AP news agency.
"We have to leave the tar sands oil in the ground. That's the only solution if we are going to save the planet," protester Martin Springhetti told AP.
TransCanada said the protesters were ignoring the jobs the pipeline would create.
"What these millionaire actors and professional activists do not seem to understand is that saying no to Keystone means saying yes to more conflict oil from the Middle East and Venezuela filling American gas tanks," said TransCanada spokesman James Millar.
The US State Department is handling public consultations on the project as the pipeline would cross the border with Canada, but the White House has made clear President Obama will influence the final outcome.
The decision as to whether to allow the project is fraught with political risk for Mr Obama, reports the BBC's Zoe Conway in Washington.
If he rejects it, he could be accused of destroying jobs. But allowing it to go ahead could lose him the support of activists who helped propel him to the White House.
Some predict that the decision will be delayed until after next year's presidential election.
Last week the State Department indicated it might not reach a decision on whether to issue a permit for the pipeline by the end of 2011, as was originally planned.