CANNON BALL, N.D. - Authorities have begun arresting some Dakota Access pipeline protesters at a camp the demonstrators set up on private land in the path of pipeline construction.

An Associated Press reporter says law enforcement officers formed a horseshoe-like loop around the camp in North Dakota.

Several protesters were arrested on the perimeter of the camp as authorities slowly moved in.

Police and soldiers launched the operation at midday to oust the protesters, a day after they refused to leave voluntarily.

Many of the protesters were openly defying the authorities, yelling at them as they approached. Others were taking part in prayer circles.

The operation that commenced at midday involved dozens of officers in riot gear, some of them armed, along with trucks, police cars, military Humvees and buses. At least two helicopters and a fixed-wing airplane monitored the operation from the air.