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.