Federal officials said the roundup is needed because the 850 square miles of land is overpopulated and could become unlivable to wildlife and livestock within four years.
Bureau of Land Management spokeswoman JoLynn Worley said the agency began gathering horses Monday in the eastern portion of the Black Rock Range, a stretch of mountains more than 100 miles north of Reno, Nev.
A contractor was using two helicopters under BLM supervision to move the horses to corrals, Worley said. The animals were then being trucked to Fallon, Nev., for immunizations and veterinary care, she said.
Worley said officials won't know how many horses were captured on Monday until early Tuesday. She said the agency would likely be in the range for one week to 10 days - with a goal of capturing 250 mustangs - before moving on to the next of five areas.
Long-term plans call for the mustangs to be placed for adoption or sent to holding facilities in the Midwest. The agency said a facility in Reno was full of adoptable horses, making it unclear when the animals gathered in the latest capture could be put up for adoption.
Horse defenders say the use of helicopters to drive horses to corrals is inhumane and risks their injury and death. Opponents also contend winter roundups expose horses to the risk of respiratory illness.
Program Director Suzanne Roy of In Defense of Animals said the group questions the timing of the roundup and methods that prevent public monitoring of the roundup.
"It just all smells bad," she said. The California-based group has had trouble getting White House and other federal officials to work through their complaints during the holidays, she added.
About 30 protesters gathered Sunday at the entrance to Red Rock Canyon National Conservation Area west of Las Vegas, waving down motorists and holding placards.
The group also planned to demonstrate Wednesday outside the San Francisco office of U.S. Sen. Dianne Feinstein, D-Calif., who protesters hope will sympathize with their calls for a moratorium on wild horse roundups. Other protests were being planned for Wednesday in Chicago and Denver.
Worley said the agency planned to take reporters near the corralling sites on Wednesday and was working through details for public viewing areas as the roundup moves to different areas.
The roundup was to include horses from five federally managed areas in the Calico Mountains complex.
A September count showed more than 3,040 wild horses were living in the area, about three times the land's capacity, federal officials said.
Without the roundup, the horse population in the area would grow by 20 percent to 27 percent annually, passing 6,000 mustangs within four years, according to BLM. At that point, wildlife and livestock wouldn't have enough water or forage.
The roundup is part of the Bureau of Land Management's overall strategy to remove thousands of mustangs from public lands across the West to protect wild horse herds and the rangelands that support them. The bureau estimates about half of the nearly 37,000 wild mustangs live in Nevada, with others concentrated in Arizona, California, Oregon, Utah and Wyoming.