NBC NEWS - In what may be a final show of defiance, the last of the Dakota Access pipeline protesters set some of their tents and teepees ablaze Wednesday as the deadline loomed to depart their encampment near the Standing Rock Sioux reservation.

Calling it a "ceremonial act," the remaining 300 or so pipeline opponents — down from the thousands at the height of the sometimes violent protests — set the structures ablaze as the camp leaders were informed that anybody still at the camp after 2 p.m. local time would be subject to arrest.

Thick black smoke billowed as snow fell on the remaining holdouts.

North Dakota officials, who say the demonstrators have to go because of the impending spring floods, have set up a travel assistance center and are providing bus fare, food and hotel vouchers to get them out of the area. The first buses from the camp to the city of Bismarck began rolling out at 9 a.m. CT.

But a huge police presence remained at the scene just in case the most hardcore protesters — who consider themselves "water protectors" — make a last stand.

The Sioux tribes, backed by environmental activists, have been protesting since August against plans by Texas-based Energy Transfer Partners to build a 1,170-mile pipeline that would transport 470,000 barrels of oil a day across their territory.

Arguing that an oil spill would contaminate the reservation's water supply and destroy sacred sites where their ancestors are buried, the Native Americans and their allies furiously opposed a pipeline proposal that was backed by the state's Republican leadership.

Defenders of the $3.7 billion project, which is already 70 percent completed, claimed it would inject millions of dollars into local economies and create anywhere from 8,000 to 12,000 construction jobs.

In the months that followed, there were repeated clashes between demonstrators and police that left hundreds injured — as well as demonstrations across the country in solidarity, like the National Day of Action on Nov. 15.

The Obama Administration gave the Native Americans a short-lived victory on Dec. 4 with announcement that it would not grant the energy company an easement to continue the construction.

"This is something that will go down in history, and I know that it's a blessing for all indigenous peoples," Standing Rock Sioux Chairman Dave Archambault II told NBC News at the time.

But on his second day in office, President Donald Trump signed a memorandum giving the pipeline project the green light. And on Feb. 7, the U.S. Army Corps of Engineers, approved the easement.

The pipeline will connect the oil producing areas in North Dakota to a crude oil terminal in Patoka, Illinois.