NEAR CANNON BALL, N.D. — Horseback riders, their faces streaked in yellow and black paint, led the procession out of their tepee-dotted camp. Two hundred people followed, making their daily walk a mile up a rural highway to a patch of prairie grass and excavated dirt that has become a new kind of battlefield, between a pipeline and American Indians who say it will threaten water supplies and sacred lands.

The Texas-based company building the Dakota Access pipeline, Energy Transfer Partners, calls the project a major step toward the United States’ weaning itself off foreign oil. The company says the nearly 1,170-mile buried pipeline will infuse millions of dollars into local economies and is safer than trucks and train cars that can topple and spill and crash and burn.

But the people who stood at the gates of a construction site where crews had been building an access road toward the pipeline viewed the project as a wounding intrusion onto lands where generations of their ancestors hunted bison, gathered water and were born and buried, long before treaties and fences stamped a different order onto the Plains.

People have been gathering since April, but as hundreds more poured in over the past two weeks, confrontations began rising among protesters, sheriff’s officers and construction workers with the pipeline company. Local officials are struggling to handle hundreds of demonstrators filling the roads to protest and camp out in once-empty grassland about an hour south of Bismarck, the state capital.