Hollywood actor Mark Ruffalo joined about 5,000 other demonstrators, including Nobel Prize laureate Jody Williams, in a march around the White House on Sunday to denounce the proposed Keystone XL oil pipeline.

The $7-billion pipeline would carry crude from the Alberta oilsands through six U.S. states to refineries on the Gulf Coast.

Critics say the project poses grave environmental risks in the event of a spill because it would meander through sensitive ecological territory. Backers say the oil is badly needed by petroleum-hungry markets. 

CBC News spoke to Ruffalo on Sunday from Washington, D.C., where protesters were trying to persuade U.S. President Barack Obama to withhold approval for the pipeline.