CANNON BALL, N.D. (Reuters) - All but a few dozen of the last holdouts from a months-long mass protest against a proposed oil pipeline in North Dakota peacefully vacated their riverside camp as an eviction deadline passed on Wednesday.

Protesters march, with a structure burning in the background, on the outskirts of the main opposition camp against the Dakota Access oil pipeline near Cannon Ball, North Dakota, U.S., February 22, 2017. REUTERS/Terray Sylvester

“We’ve very firm that the camp is now closed,” Governor Doug Burgum, a Republican, told an evening news conference.

Following Wednesday’s exodus, Burgum estimated there were 25 to 50 protesters left. He said they were still free to leave voluntarily so long as they did not interfere with cleanup crews scheduled to enter the site at 9 a.m. on Thursday.

The encampment has stood since August on U.S. Army Corps of Engineers property at the edge of the Standing Rock Sioux Reservation near Cannon Ball, North Dakota, about 40 miles south of Bismarck, the state capital.

Protesters calling themselves “water protectors” have rallied there against plans to route the Dakota Access Pipeline beneath a lake near the reservation, saying the project poses a threat to water resources and sacred tribal sites.

Dubbed the Oceti Sakowin camp, the site became a focal point for U.S. environmental activists and Native Americans expressing indigenous rights, drawing some 5,000 to 10,000 protesters at the height of the movement in early December.

Most have drifted since away, as tribal leaders urged people to leave due to harsh winter weather, while pressing their opposition to the pipeline in court. Roughly 300 demonstrators had remained until this week.

Protesters and police have clashed multiple times since August, with more than 700 arrests tallied.

On Wednesday authorities appeared intent on avoiding clashes, though 10 arrests were made as protesters confronted police in riot gear on a highway outside the camp entrance before the officers retreated around nightfall.

Slideshow ( 18 images )

President Donald Trump has pushed for completion of the pipeline since he took office last month, signing an executive order that reversed an Obama administration decision and cleared the way for the $3.8 billion project to proceed.

Two tribes earlier this month lost a legal bid to halt construction. The pipeline is due to be complete and ready for oil by April 1, according to court documents filed Tuesday.

DEADLINE ON THE RIVER

Burgum and the Army Corps of Engineers had set Wednesday’s deadline for protesters to leave, citing hazards posed by impending spring floods along the Cannonball River.

The governor said the handful of demonstrators who remained needed to make way for crews set to expand a cleanup that began weeks ago to remove mounds of garbage, debris, human waste and dozens of abandoned vehicles.

At least three dozen protesters could be seen gathering near the camp entrance as the afternoon eviction deadline passed, and a few dozen others were believed lingering elsewhere at the site. Some vowed to stay put.

“I feel as though now is the time to stand our ground,” said Alethea Phillips, 17, a demonstrator from Michigan who had spent three months at the camp.

Chase Iron Eyes, a Standing Rock Sioux member, said closing the camp would not dampen his determination.

“You can’t arrest a movement. You can’t arrest a spiritual revolution,” he said.

Activists set off fireworks on Wednesday morning, and as freezing rain and snow fell, some demonstrators ceremonially burned tents and other structures at the camp.

State officials said protesters had set about 20 fires, and that two youngsters - a 7-year-old boy and a 17-year-old girl - were taken to a Bismarck hospital for burns after two explosions occurred, the governor said.

Authorities have set up an assistance center to provide departing protesters with food, water and medical check-ups, as well as a voucher for one night’s hotel stay and a bus ride home.