Three of Nevada rancher Cliven Bundy's sons and what they claim are 150 militia members have occupied a federal building in eastern Oregon in order to keep two local ranchers out of prison, according to local reports.

The group is believed to be heavily-armed.

According to The Oregonian, the group seized the headquarters building at Malheur National Wildlife Refuge about 50 miles outside Burns, Ore. The remote facility was closed and unoccupied at the time.

Bundy and his supporters were in Oregon after two men were scheduled to go to prison on Monday for setting fires on federal land, according to a report by Oregon Public Broadcasting. The case has caused a stir in eastern Oregon because the two men were charged under anti-terrorism laws.

The two men, Dwight and Steve Hammond, appealed their cases multiple times, the report stated. They were required to report to prison on Monday.

The militia groups say the Hammond's were illegally prosecuted for setting what they claim was a controlled fire because the federal government has no authority at the county level.

"The facility has been the tool to do all the tyranny that has been placed upon the Hammonds," Ammon Bundy told the Oregonian.

Cliven Bundy, a rancher who engaged in an armed standoff with federal officials due a dispute he had with the Bureau of Land Management, was not present.

Earlier Saturday the Bundy brothers and an estimated 300 protesters marched in through Burns, Oregon, about 50 miles from the building they seized.

Ammon Bundy on Saturday posted a video on his Facebook page asking "patriots" from across the country should to report to the refuge with weapons.

An investigative reporter for The Oregonian tweeted that Ammon Bundy wants to use the federal refuge as a militia base for years to come and called for anti-government supporters from around the country to come to the area.

It's unknown if the men are armed, but Cliven Bundy told John Sepulvado with Oregon Public Broadcasting that the militiamen have enough food supplies to last "a while."

In an interview with The Oregonian, Ammon Bundy refused to rule out violence if law enforcement officials try to intervene.

"We're planning on staying here for years, absolutely," he told The Oregonian. "This is not a decision we've made at the last minute."

Ammon Bundy said he met with local ranchers in the area that refused to take part in the seizure of the federal building, but decided to go ahead with the effort anyway.

He said he's calling out to other militiamen around the country to join the cause.

"We hope they will grab onto this and realize that it's been happening," he said.

Cliven Bundy gained international notoriety in 2014 because he refused to pay more than $1 million he owed the federal government for grazing cattle on federal property. Anti-government militias flocked to the Nevada ranch and engaged Bureau of Land Management and other federal agencies in an armed standoff for weeks.