An elderly woman is escorted to a transport van after being arrested by law enforcement at the Oceti Sakowin camp as part of the final sweep of the Dakota Access pipeline protesters in Morton County on Thursday in North Dakota. Authorities said they arrested 46 people.

CANNON BALL, N.D. — Authorities on Thursday 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 ½ hours for about 220 officers and 18 National Guardsmen to methodically search the protesters’ temporary homes. 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.

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.

Most protesters left peacefully Wednesday, when authorities closed the camp, but some stayed overnight in defiance of the government order.