After nearly a year of occupying North Dakota prairie land to block the route of a controversial oil pipeline, many of the camp's holdouts finally marched out Wednesday to meet an evacuation deadline.

Some 10 activists who had remained after the 2:00 pm (2000 GMT) deadline passed were arrested, according to the North Dakota Joint Information Center.

Earlier this month, President Donald Trump signed an executive order to revive the pipeline project. After the final permit was issued, construction on Dakota Access began almost immediately.

Native Americans and their supporters began leaving the federal land -- which was occupied by a population that swelled into the thousands at times -- singing traditional songs and banging drums.

Many opposed to the pipeline say it threatens the drinking water of the Standing Rock Sioux tribe. The pipeline's operator, Energy Transfer Partners, insists it is safe, with high-tech systems in place to prevent environmental catastrophe.

State and tribal authorities planned to begin coordinated efforts to clean up the camp, removing garbage, structures, vehicles and other debris, in anticipation of seasonal flooding in the area.

Without the cleanup, the authorities said local waterways could be contaminated. More than 230 truckloads of debris had been cleared out as of Monday, officials said.

Campers burned some structures on their way out of the camp, in what they said were ceremonial rituals.

They set approximately 20 fires, the authorities said, adding that at least two explosions also took place after which a seven-year-old boy and a 17-year-old girl were taken to a hospital with burns.

Some protesters had informed law enforcement that they would engage in passive resistance and expected to be arrested, Tom Iverson of the North Dakota Highway Patrol told local television station KFYR.

"People were free to leave," he said, adding that the evacuation was intended to "avoid an ecological disaster."

State authorities were also offering protesters bus fare to return home and hotel lodging for one night.

Native Americans and others began protesting at the camp starting last April, in opposition to the 1,172-mile (1,886-kilometer) oil pipeline. Its route runs under land the Standing Rock Sioux consider sacred and under the Missouri River, which is the source of drinking water for the tribe's reservation.

The tribe filed a motion in federal court last week asking that pipeline construction be halted until a full environmental impact review is completed.