CANNON BALL, N.D. (Reuters) - Native American leaders vowed on Saturday to protest through the winter against a North Dakota oil pipeline they say threatens water resources and sacred lands, and are weighing lawsuits over police treatment of arrested protesters.

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A group of at least 200 Native American demonstrators meanwhile returned to the scene of an earlier confrontation with police to stage a peaceful ceremonial prayer vigil near the town of Cannon Ball, at the edge of the Standing Rock Sioux Reservation.

A smaller crowd of 25 to 50 rallied on the grounds of the state capitol in Bismarck, about 30 miles to the north, in a separate protest of the $3.8 billion Dakota Access Pipeline, police said. No arrests were reported at either location.

At a news conference in Mandan, just outside Bismarck, Standing Rock Sioux Chairman Dave Archambault II said he and other tribal leaders were devising ways to furnish food, heat and shelter for protests to continue through the cold-weather months.

“We’re just working through some technical details as far as where the land is, and the type of land that can be used for some permanent structures,” Archambault told reporters.

At least 10 shelters were being readied on tribal land against temperatures that can plunge to less than 35 degrees below zero Fahrenheit (-37 Celsius) for days at time, he said.

“Let’s reroute the pipeline. It doesn’t have to put our water at risk,” said Archambault, who was flanked by Cheyenne River Sioux Chairman Harold Frazier.

The planned 1,172-mile (1,885-km) path of the pipeline, the project of a group of companies led by Energy Transfer Partners LP, would skirt the Standing Rock reservation by about a half mile. But the Standing Rock tribe and environmental activists say it threatens water supplies as well as sacred Native American sites.

Supporters say the pipeline, construction of which was halted by the federal government in September, offers the fastest and most direct route for bringing Bakken shale oil from North Dakota to U.S. Gulf Coast refineries.

More than 400 protesters have been arrested in protests against the pipeline since Aug. 10 that have attracted support from such actors and celebrities as Mark Ruffalo, Shailene Woodley, Susan Sarandon and Chris Hemsworth.

Archambault said his tribe may pursue a class-action over police tactics on Thursday.

Officers in riot gear swept through a protester camp on private land using pepper spray, bean bag rounds and an audio cannon against demonstrators who refused to leave. At least 142 people were arrested on Thursday and Friday.

The Morton County Sheriff’s Department has said some protesters set fire to roadblocks and threw rocks, bottles and homemade gasoline bombs at officers.

In an apparent easing of tensions on Saturday, sheriff’s deputies allowed activists under escort to retrieve personal belongings left behind at the protest site in Thursday’s raids.

Separately, a group of 200 to 300 protesters led by Chief Arvol Looking Horse, spiritual leader of the Sioux nation, marched from a nearby campground to a police barricade, where they held a quiet prayer ritual and ceremonial dancing.

The chief crossed the barricade to shake hands with a number of police officers, and the group quietly left after about three hours.