The last stretch of the $3.7 billion project involves laying pipe under a Missouri River reservoir, which the Standing Rock Sioux say threatens their land and water.
In downtown Los Angeles, hundreds joined the nationwide call to action to stop the project. Among the crowd were actresses Marisa Tomei and Alfre Woodard.
"I'm here to stand with the people of Standing Rock and to support the First Nations people," Tomei said. "Protect the water and not let the pipeline go through their sacred lands."
Other protesters noted that if the oil pipeline under the river ruptures, it could ruin the water supply.
The companies behind the pipeline argue it will provide a safer, more efficient way to transport oil from North Dakota to the Midwest. The Army Corps of Engineers delayed granting an easement to complete the pipeline and now companies backing the project are asking a U.S court to intervene.