Photo: The Canadian Press

Authorities have cleared a protest camp where opponents of the Dakota Access oil pipeline had gathered for the better part of a year, searching tents and huts and arresting dozens of holdouts who had defied a government order to leave.

It took 3 1/2 hours for about 220 officers and 18 National Guardsmen to methodically search the protesters' temporary homes on Thursday. Authorities said they arrested 46 people, including a group of military veterans who had to be carried out and a man who climbed atop a building and stayed there for more than an hour before surrendering.

Native Americans who oppose the $3.8 billion pipeline established the Oceti Sakowin camp last April on federal land near the Standing Rock Indian Reservation to draw attention to their concerns that the project will hurt the environment and sacred sites — claims Dallas-based pipeline developer Energy Transfer Partners disputes. The camp gained increased attention starting in August after its population had grown and authorities made their first arrests. At its height, the camp included thousands of people, but the numbers had dwindled during the winter and as the fight over the pipeline moved into the courts.

The Army Corps of Engineers said it needed to clear the camp ahead of spring flooding, and had ordered everyone to leave by 2 p.m. Wednesday. The agency said it was concerned about protesters' safety and about the environmental effects of tents, cars, garbage and other items in the camp being washed into nearby rivers.

"The battleground has shifted to the legal courts and the court of public opinion," said ​American Indian activist Chase Iron Eyes, referring to lawsuits filed by tribes and an effort planned by the Lakota People's Law Project to rally lawmakers and others in Washington, D.C., to their cause.