CANNON BALL, N.D. >> Police in full riot gear began arresting Dakota Access pipeline opponents who remained in a protest camp in North Dakota today in defiance of orders to leave.

Most protesters left peacefully Wednesday, when authorities closed the camp on Army Corps of Engineers land in advance of spring flooding, but some refused to go.

Eighteen National Guardsmen and dozens of law officers entered the camp from two directions shortly before midday today, along with several law enforcement and military vehicles. A helicopter and airplane flew overhead.

Officers checked structures and began arresting people, putting them in vans to take to jail. The number of arrests wasn’t immediately known.

The operation began shortly after authorities said Corps officials had met with camp leaders. They didn’t divulge the outcome of those talks.

The camp — known as Oceti Sakowin — near the Standing Rock Indian Reservation has since August been the main site for demonstrators trying to thwart construction of the final section of the $3.8 billion pipeline. The Standing Rock Sioux and Cheyenne River Sioux, whose reservation is downstream, say Dakota Access 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.

Police also had a SWAT vehicle on hand today in case of what Highway Patrol Lt. Tom Iverson described as a worst-case “SWAT scenario” — an armed person barricading themselves in a structure in the camp.

American Indian elders have told police there are people willing to resort to drastic measures to stay in the camp, Iverson said. Similar sentiments have been expressed by protesters on social media, Iverson said.

“We’re doing everything we can to avoid that kind of a situation,” he said. “We don’t want it to reach a flash point, but at some point, enough is enough.”

At its peak, the camp was home to thousands of protesters. Gov. Doug Burgum estimated Wednesday night that as many as 50 people remained in the camp. Police early today said an additional 15 crossed a frozen river and entered the camp on foot.

Before authorities moved in, 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 at midday to transport anyone to Bismarck, where officials were doling out basic necessities, along with hotel and bus vouchers.

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. The Corps had warned that the protesters need to leave the site before the spring melt floods the land and spreads debris from the camp downriver.

Early Wednesday, protesters burned some wooden structures on site in what they described as a leaving ceremony. Authorities said about 20 fires were set and a 7-year-old boy and a 17-year-old girl were taken to hospitals to be treated for burns.

Shortly before the 2 p.m. deadline to leave, about 150 people marched out of the soggy camp, singing and playing drums as they walked down a highway, carrying an American flag hung upside-down.