By Peter Henderson and Edward Tobin

(Reuters) - Anti-U.S. government groups called on supporters to "stand by" on Wednesday after one protester was killed and eight others were arrested in the month-long occupation of an Oregon wildlife reserve over federal control of land. 

The stand-off between U.S. authorities and an armed group of men turned violent on Tuesday after officers stopped a car carrying protest leader Ammon Bundy and others near the Malheur National Wildlife Refuge. Activists said Robert LaVoy Finicum, a rancher who acted as a spokesman for the occupiers, was killed.

There were no details on what set off the shooting. The Federal Bureau of Investigation said authorities would hold a news conference on Wednesday at 9:30 a.m. PST (1730 GMT) in Burns, a town near the refuge.

Amid concerns that Finicum's killing could escalate violence, the Pacific Patriots Network, Oath Keepers and the Idaho III% - groups sympathetic to the occupiers - said in a joint statement they were issuing an immediate “stand by” order for their followers.

"During this time, cooler heads must prevail," the statement said. "We do not wish to inflame the current situation and will engage in open dialogue until all of the facts have been gathered."

Anti-militia sentiment also lit up social media, making #OregonStandoff among the top trending hashtags.

Authorities said the new security involves a series of checkpoints along key routes into and out of the refuge and was made out of an "abundance of caution" to protect the public and law enforcement. Only ranchers who own property in the area will be allowed in and anyone coming out of the refuge will have to show identity and have their vehicle searched.

A law enforcement official said authorities setting up a perimeter around the refuge "remain hopeful" that those on the compound will now depart peacefully.

"PEACEFUL RESOLUTION"

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One of the remaining occupiers at the reserve, Jason Patrick, told Reuters by phone they would stay until the "redress of grievances."

“I've heard 'peaceful resolution' for weeks now and now there’s a cowboy who is my friend who is dead – so prepare for the peaceful resolution,” Patrick said.

The Malheur takeover, which started Jan. 2 with at least a dozen armed men, was a flare-up in the so-called Sagebrush Rebellion, a decades-old conflict over federal control of millions of acres in the West. Protesters say they are defending the Constitution. Bundy's father, Cliven, was a key figure in a 2014 armed standoff over unpaid grazing fees in Nevada.

The arrests have angered anti-government protesters across the country, said Mike Vanderboegh, a gun-rights activist active in self-proclaimed militia circles. "It's all I can do to keep people from going and shooting feds right now," he told Reuters.

Vanderboegh said he believed the FBI had acted too quickly to end a situation that was already on its way to a peaceful resolution.

Federal officials said Tuesday they had probable cause to arrest Finicum, who told NBC News earlier this month that he would rather die than be detained.

In an interview on Monday with the Oregonian newspaper, Finicum said federal authorities had increased manpower around the refuge and stepped up their airplane and drone surveillance. There also was a change of attitude, he said.

"We used to could walk up to them and talk with the FBI agents in a friendly manner ... but the tenor has changed," Finicum said. "They do not intend on losing here. And we do not intend on giving it back to them."

Those arrested face federal charges of conspiracy to use force, intimidation or threats to impede federal officers from discharging their duties, the FBI said.

Patrick likened Finicum’s death to that of Tamir Rice, an unarmed 12-year-old black youth fatally shot by Cleveland police outside a recreation center in 2014. The officers were not charged.

"The government can kill who they want for whatever reason they want with impunity," Patrick said.

Many Twitter users expressed satisfaction about the arrests. David Plotnik (@davidkippy) tweeted: “Finally, the #Bundy militia get what they deserve. They cannot set a good example for all the radical #republicans out there. #democrats."

(Reporting by Curtis Skinner in San Francisco, Dan Whitcomb in Los Angeles and Jonathan Allen and Melissa Fares, Andy Sullivan and Julia Edwards; Writing by Edward Tobin in New York; Editing by Bill Trott)