"Turn in your arms; the government will take care of you," read the words above and below a photo of three Native Americans, one of whom is holding a rifle.
Denver-based Lamar Advertising, which owns the ad space, says the group that purchased the signs wishes to remain anonymous.
"I think it's a little bit extreme, of course, but I think people are really worried about their gun rights and what liberties are going to be taken away," Lamar Advertising rep Matt Wells told the Greeley Tribune.
"I thought it was pretty cowardly that someone would put something like that up and spend the money for a billboard but didn't have the courage to put their name on it," said Greeley resident Maureen Brucker, an honorary member of the Pine Ridge Indian Reservation, who added that the billboard brings to mind the infamous Wounded Knee Massacre, which took place at Pine Ridge.
"It wasn't just about our guns," notes Irene Vernon, a Native American who chairs the ethnic studies department at Colorado State University.
Still, despite the outcry over cultural insensitivity and historical revisionism, some still support the tactic.