By John L. Smith

LAS VEGAS (Reuters) - Six men accused of taking part in a standoff at the Nevada property of rancher Cliven Bundy aimed rifles at law enforcement during the confrontation, prosecutors said on Thursday during opening remarks in the first of three trials stemming from the land-rights dispute.

Defense attorneys countered that the defendants, who were among hundreds of people who gathered at Bundy's ranch near Bunkerville, 75 miles (120 km) northeast of Las Vegas, were standing up for rights protected by the U.S. Constitution.

"They instilled fear in the officers," Assistant U.S. Attorney Steven Myhre told a federal court jury in Las Vegas. "We will prove to you it was their goal to show force."

Bundy’s decades-long fight with the U.S. government over unpaid grazing fees came to a head in March 2014 when Bureau of Land Management officials attempted to enforce a court-ordered roundup of his cattle.

The rancher, who has achieved celebrity status in the West over his opposition to government land policy, is scheduled to stand trial later this year along with other defendants considered leaders of the standoff.

The six defendants in the first trial have been described by prosecutors as Bundy's "gunmen and followers," who by mid-April had arrived from Arizona, Idaho and Montana armed with assault rifles and other firearms. They are accused of a conspiracy to recover Bundy's cattle “by force, threats and intimidation.”

Gregory Burleson, O. Scott Drexler, Todd Engel, Richard Lovelien, Eric Parker, and Scott Stewart have all pleaded not guilty. During opening statements, their attorneys sought to cast the trial as a clash of legal and political philosophies over the management and use of public lands in the West.

"This case, folks, is about standing up for what you believe in. Nothing more, nothing less," said attorney Richard Tanasi, who represents Stewart.

Although the six defendants are considered by the court to be the lowest level of the 17 defendants in the three separate trials, prosecutors hope to benefit from a series of photographs of them heavily armed, some published widely.

The federal government controls 85 percent of the land in Nevada, the most of any state. While prosecutors have requested that the judge focus the trial on the standoff, the defense is seeking to broaden it to include contested federal land management.

With some 150 witnesses expected to testify, the trial could last more than two months.

(Reporting by John L. Smith; Editing by Dan Whitcomb and Peter Cooney)