CANNON BALL, N.D. — Law enforcement officers dressed in riot gear fired bean bags and pepper spray Thursday as they tried to clear protesters from a camp on private land in the path of the Dakota Access oil pipeline.

North Dakota State Emergency Services spokeswoman Cecily Fong said the officers were responding to “aggressive” tactics by protesters, including some throwing rocks at officers and threatening them.

A male protester was holding his leg after an Associated Press reporter at the scene described hearing a loud boom. A protester with a medic bag tended to the man’s leg, and he was up and walking a short time later. Fong said she wasn’t aware of any serious injuries to officers or protesters.

More than 20 protesters were arrested, Fong said. Several were led away and put in trucks, including at least one handcuffed.

The confrontation marked a major escalation of a protest that has raged for months. Opponents of the pipeline moved in over the weekend to establish a camp on private land where the developer was working to complete the 1,200-mile pipeline designed to carry oil from western North Dakota to Illinois. The route of the pipeline skirts the Standing Rock Reservation and the tribe says it could endanger water supplies and disturb cultural sites. The state of North Dakota says no sensitive cultural sites have been found in the area.

Police and soldiers driving trucks, military Humvees and buses began the operation to clear the camp at midday and formed a horseshoe-like loop once they reached the camp, where about 200 protesters were awaiting them – some defiant and other praying.

Officials used a loudspeaker to warn protesters to move out. Two helicopters and an airplane monitored the operation from the air.

The operation to push out the protesters began a day after they had refused to leave voluntarily.

The camp is just to the north of a more permanent and larger encampment on federally-owned land which has been the main staging area for hundreds of protesters, including Native Americans from across North America, environmentalists and some celebrities.

Morton County Sheriff Kyle Kirchmeier said in a statement that the protesters’ actions “forced law enforcement to respond.”

“I can’t stress it enough, this is a public safety issue,” he said. “We cannot have protesters blocking county roads, blocking state highways or trespassing on private property.”

Fong said that once the camp is cleared, authorities will turn over the land to developer Energy Transfer Partners, which will be responsible for securing the site in the future.

But Robert Eder, a 64-year-old Vietnam War veteran from the Standing Rock Reservation, said protesters will return.

“If they take everybody to jail, there will be twice as many tomorrow, and every day that passes more will come,” he said.