Sen. Robb Hogg, a Democrat from Cedar Rapids, set up the protest in Des Moines against TransCanada's plans for a pipeline through the United States from Alberta oil fields.
On Saturday, protesters carried placards equating carbon dioxide emissions to a chemical weapon, the Des Moines Register reported. Tar sands oil, the type of crude oil designated for Keystone XL, is seen as more carbon-intensive to produce.
TransCanada needs a federal permit for the cross-border section of the pipeline planned from Alberta to Nebraska. The pipeline's route does not cross through Iowa.
U.S. lawmakers heard testimony last week about the planned pipeline and Thursday marked the fifth anniversary of TransCanada's permit application to the U.S. State Department.
Bold Nebraska, a pipeline opponent, issued a statement Sunday calling on some lawmakers to apologize to its director, Jane Kleeb. The advocacy group said Kleeb was the target of personal attacks during her testimony.