SIOUX FALLS, S.D. — North Dakota law enforcement outfitted in riot gear arrested at least 16 protesters Thursday as they cleared out illegal campsites and roadblocks set up to halt construction of the Dakota Access Pipeline.

After Wednesday's negotiations with protesters fell through, authorities from more than a dozen counties dismantled the makeshift camps and roadblocks on private land along Highway1806 and County Road 134 near Cannon Ball, N.D., removing anyone who refused to leave.

Donnell Preskey, a spokesperson with the Morton County Sheriff’s Department, said the call to action began at 11:15 a.m. Thursday.

Law enforcement officers and soldiers driving trucks, military Humvees and buses began to advance, forming a horseshoe-like loop once they reached the camp, where about 200 protesters were awaiting them — some defiant and other praying, The Associated Press reported.

Preskey said protesters requested another mediation session, but authorities said that approach hadn’t worked.

“The sheriff went to the roadblock to ask them to remove the roadblock and remove the people,” Preskey said. “They were told by the protesters and a spokesperson that they were not moving.”

Teepees, tents, and other structures made of stumps, tree limbs and tires have been on land owned by pipeline developer Energy Transfer Partners since Sunday. Authorities were communicating with protesters over loudspeakers.

Rob Keller, another spokesperson with the Morton County Sheriff’s office, said tires at the roadblocks near Highway 1806 were set on fire.

In a statement, the Morton County’s Sheriff's Office said they could not allow protesters to block county roads or state highways, or to trespass on private property. “I can’t stress it enough, this is a public safety issue,” Sheriff Kyle Kirchmeier said.

Protests at the construction site for the Dakota Access Pipeline have gone on for months in support of the Standing Rock Sioux Tribe’s fight against the pipeline.

The months-long dispute over the four-state, $3.8 billion pipeline reached a crisis point when the protesters set up camp Sunday. The area is just to the north of a more permanent and larger encampment on federally owned land where hundreds of protesters have camped for months.

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

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

Contributing: Associated Press