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 three dozen holdouts who had defied a government order to leave.

It took 3½ hours for about 220 officers and 18 National Guard members to methodically search the protesters' temporary homes and arrest people, including 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 US pipeline established the Oceti Sakowin camp last April near the Standing Rock Indian Reservation to try to thwart construction of the final section of the pipeline.

The Army Corps of Engineers, which manages the land, 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 on Wednesday, when authorities closed the camp, but some stayed overnight in defiance of the government order. 

 As police in full riot gear worked to arrest the stragglers Thursday, cleanup crews began razing buildings on the property at the confluence of the Cannonball and Missouri rivers.

Arrests a last resort

Protesters maintain the camp is on land that rightfully belongs to American Indians under old treaties. The Indigenous Environmental Network, which once had a strong presence in the camp, denounced the eviction of those who remained. Executive director Tom Goldtooth said it "is a continuation of a centuries-old practice, where the U.S. government forcefully removes Indigenous people from our lands."

Hundreds, and at times thousands, of pipeline opponents who have dubbed themselves "water protectors" camped in the area for months, often clashing with police and prompting more than 625 arrests.

The Standing Rock Sioux and Cheyenne River Sioux, whose reservation is downstream, say the pipeline threatens their drinking water and cultural sites. Dallas-based pipeline developer Energy Transfer Partners disputes that. When complete, the pipeline will carry oil through the Dakotas and Iowa to a shipping point in Illinois.

On Thursday, authorities entered the camp "cautiously and tactfully" to ensure the safety of officers and protesters, according to Highway Patrol Lt. Tom Iverson. The arrests were a last resort, he said. 

"We did not want this. Unfortunately, there were some bad actors that forced us into this position," he said. 

Only one person resisted arrest; otherwise there were no major incidents, and there were no injuries, Morton County Sheriff Kyle Kirchmeier said. 

Afterward, officers showed visible relief, smiling, shaking hands and patting one another on the back.

$1.2M cleanup

Before authorities moved in, Gov. Doug Burgum had said those remaining at the camp still had a chance to leave without facing charges. The state sent a bus to the site on Thursday to transport anyone who wished to Bismarck, N.D., where officials were doling out basic necessities along with hotel and bus vouchers.

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Frankie Tso Jr., 19, from the Navajo tribe of Arizona, confronts police on the outskirts of Oceti Sakowin, which has been the main opposition camp against the pipeline. (Terray Sylvester/Reuters)

No one took advantage of the offer Thursday, and only nine people used the centre Tuesday and Wednesday, so the centre was closed, state emergency services spokeswoman Cecily Fong said.

Corps Col. John Henderson has said the taxpayer-funded cleanup of the site could take about a month and cost as much as $1.2 million.

New camps popping up

Some of the protesters were focused on moving off federal land and away from the flood plain into other camps. New camps are popping up on private land, including one the Cheyenne River Sioux set up about 1½ kilometres from the main camp.

Nathan Phillips, a member of the Omaha tribe in Nebraska, said he was moving to a new camp in the area because "there's still work to be done." He has been in North Dakota since Thanksgiving and said he's had "four showers since."

Matthew Bishop, from Ketchikan, Alaska, packed up Wednesday and said he too was headed to a camp on private land.

"We're going to regroup and see what we can do," Bishop said.

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Protesters march, with a structure burning in the background at Oceti Sakowin. (Terray Sylvester/Reuters)

In December, the previous U.S. administration halted construction on the remaining section of pipeline pending a review, and on Jan. 21, the Standing Rock Sioux thanked supporters and asked those in the camps to clean up the land and leave within 30 days. 

But a few days later, U.S. President Donald Trump signed an executive action to move forward on construction of the pipeline.