Opponents led by American Indian drummers marched across a Missouri River bridge on Sunday. Critics fear the pipeline could pollute the environment.
Proponents say pipelines are safer than trains for transporting oil, and that the Keystone XL could create jobs and tax revenue.
South Dakota's Public Utilities Commission this week is beginning a hearing on whether to approve pipeline construction through South Dakota.
The Keystone XL would carry crude from Canada to Gulf Coast refineries, passing through several states. It also could transport some crude from the Bakken oil field.