An unpredictable new chapter in the wars over federal land use in the West unfolded on Sunday after a group of armed activists split off from an earlier protest march and occupied part of a national wildlife refuge in remote southeastern Oregon.

The activists, led by rancher Ammon Bundy, set themselves up in the Malheur National Wildlife Refuge 30 miles southeast of here, defying the organizers of a rally and march held Saturday in support of two local ranchers who are scheduled to report to federal prison Monday to serve a sentence for arson.

Some of the occupiers said they planned to stay indefinitely. Harney County Sheriff David Ward said authorities from several law enforcement organizations were monitoring the situation.

“These men came to Harney County claiming to be part of militia groups supporting local ranchers,” Ward said in a statement on Sunday. “When in reality these men had alternative motives, to attempt to overthrow the county and federal government in hopes to spark a movement across the United States.”

The occupation revealed deep divides among some Western ranchers who want freer rein over federal lands but are split on whether to achieve those goals peacefully or more confrontationally.

Organizers of Saturday’s rally said several hundred protesters marched through Burns, a ranching town of fewer than 3,000 residents, in a show of support for Dwight Hammond, 73, and his son Steven Hammond, 46, who after decades of clashes with the federal government were sentenced in October to five years in prison.

But, at the rally’s end, Ammon Bundy and an estimated dozen supporters declared they would take up arms and occupy a federal refuge building in protest. Amanda Peacher, a reporter for Oregon Public Broadcasting, reported that the men had entered a building at the refuge that was unstaffed over the weekend.