Police in full riot gear have made multiple arrests as they began clearing the Dakota Access pipeline protest camp in North Dakota.
Most protesters left peacefully on Wednesday, when authorities closed the camp on Army Corps of Engineers land, but some refused to go.
Eighteen National Guardsmen and dozens of law officers entered the camp from two directions shortly before midday on Thursday, along with several law enforcement and military vehicles.
Officers checked structures and began making arrests, with an unspecified number of protesters detained.
The operation began shortly after authorities said Corps officials had met with camp leaders.
The camp - known as Oceti Sakowin - near the Standing Rock Reservation has since August been the main site for demonstrators trying to thwart construction of the final section of the 3.8 billion dollar 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.
Native American elders have told police there are people willing to resort to drastic measures to stay in the camp, Highway Patrol Lt Tom Iverson said. Similar sentiments have been expressed by protesters on social media, he added.
At its peak, the camp was home to thousands of protesters. Governor Doug Burgum estimated on Wednesday night that as many as 50 people remained in the camp. Police said an additional 15 had crossed a frozen river and entered the camp on foot early on Thursday.
Before authorities moved in, Mr 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 giving out basic necessities, along with hotel and bus vouchers.
Corps Col John Henderson said the taxpayer-funded clean-up of the site could take about a month and cost as much as 1.2 million dollars. 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 on 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 seven-year-old boy and a 17-year-old girl were taken to hospital to be treated for burns.
Shortly before the 2 pm deadline to leave, about 150 people marched out of the soggy camp carrying an American flag hung upside down.