Anti-government militants who invaded a federal wildlife refuge in Oregon conspired to intimidate government workers and steal property in a heavily armed protest that is not protected by the U.S. Constitution, prosecutors said on Tuesday.

Assistant U.S. Attorney Geoffrey Barrow told a packed federal courtroom in downtown Portland that during the January takeover, the conspirators, many wearing camouflage and toting rifles, practiced shooting drills and hand-to-hand combat together at the Malheur National Wildlife Refuge. They also had a stockpile of some 15,000 rounds of ammunition.

"They moved as a military force," Barrow said, adding later: "We all have a right to bear arms. This is a case about what the defendants did with those firearms."

Barrow's remarks came during opening statements in the trial of ranchers Ammon and Ryan Bundy and five other limited-government activists who led the 41-day takeover of the refuge that began on Jan. 2.

The defendants pleaded not guilty to conspiracy to impede federal officers from doing their jobs through intimidation, threats or force, possession of firearms in a federal facility and theft of government property.

The takeover at Malheur was the latest flare-up in a decades-old conflict over federal control of millions of acres of public land in the West.

The Bundy brothers have been at the forefront of that movement and stood by their father, Cliven Bundy, at his Nevada ranch in a 2014 armed standoff with authorities over enforcement of federal grazing rights.

Marcus Mumford, a lawyer for Ammon Bundy, told the court the intent of the peaceful protest was to draw attention to the federal government's illegal control and mismanagement of public lands, not to intimidate federal employees.

He said Bundy was attempting to invoke a provision of the law known as adverse possession to retake land improperly seized by the federal government many decades ago.