State officials counter the action is necessary to protect motorists from becoming road kill.
With a colt and full-grown Mustang in their midst, about 40 protesters gathered on the grass and sidewalks outside the Nevada Legislature, where they held signs and rallied support from passing vehicles.
"We're not saying stop the roundup. We're saying stop the slaughter. Work with us," said Bonnie Matton of the Wild Horse Preservation League.
The advocates say the cancellation in 2009 of cooperative agreements with horse advocacy groups under the previous administration of Gov. Jim Gibbons exacerbated the problem of horses on the highways.
"We worked a decent, good wild horse program in the past," said Shirley Allen of Dayton. "It didn't cost taxpayers a dime. But since they stopped working with advocates — there's not a lot of alternatives they're leaving us."
Allen, who is with a group called Least Resistance Training Concepts that helps teach gentling techniques to owners of horses, mules and donkeys, spoke to The Associated Press while being nuzzled by "Mystic Diamond," a four-month old colt she took in when it was days old after it was found along a road near Virginia City.
Ed Foster, spokesman for the Department of Agriculture, said the removal of horses is essential for public safety.
Over the past three months, more than 35 horses have been hit on three rural highways in Lyon and Storey counties located roughly around Silver Springs and Virginia City.
"We've become a dead horse removal agency," Foster said.
No one has been seriously injured, but officials fear it's just a matter of time.
"The bottom line here, we have a duty to the citizenry of the state to protect them from a horse going through the windshield of their car," Foster said.
"I'm not saying that their hearts are not in the right place," he said of the advocates. "We perceive this to be a serious public safety issue and we just don't want anyone to get hurt over this."
He said in one accident, a horse was struck by a school bus. No children were on board. In another, a car became high centered on the animal's body.
Unlike wild mustangs protected by federal law and managed by the U.S. Bureau of Land Management, wandering horses on state lands are considered strays — abandoned by their owners — with no legal protections against slaughter.
The agency set out passive traps late last week to lure the animals to captivity.
"That's not working very well. We've had them up three days and haven't caught a one," Foster said, adding that people may be releasing the animals or shooing them away.
Once caught, the state will take photos of the animals and advertise them in local newspaper to give possible owners a chance to reclaim them. After that, they will be sold at auction at a state prison "and anyone with a checkbook can buy one," Foster said.
Horse advocates say that will invite "kill buyers," intent on selling the animals for slaughter.
Sheila Schwadel of Douglas County, a member of Alliance of Wild Horse Advocates, walked the protest line leading her 12-year-old Mustang, "Shadow," who munched on grass near the roadway with a sign advertising a price of 60 cents a pound painted on his side. That's the amount critics of the roundup say slaughter horses sell for at auction.
Foster said the agency has not closed the door on working with advocacy groups in the future.
"But right now we have an acute public safety problem," he said.