The Bureau of Land Management intends to remove about 80 per cent of the wild horses occupying five herd management areas known as the Calico complex.
It is understood about 250 horses are expected to be removed from the targeted area before the musterers move on.
Wild horse advocates have criticised the muster, branding it unnecessary and inhumane.
The bureau says it is necessary because the range is overpopulated and it cannot support the horse numbers.
Two helicopters are being used in the operation. Captured animals are being taken to Fallon, Nevada, for veterinary checks.
Most will end up long-term holding facilities in the Midwest. More than 30,000 horses are already held in captivity by the bureau.
Several protests have been held by wild horse advocates.
Las Vegas realtor and wildlife artist, Arlene Gawne, involved in organising one of the protests, says: "I am so mad, I won't take it anymore! It is time for the public to stand up to the bureau and the Secretary of the Interior, Ken Salazar, and say no more taxpayer dollars for inhumane roundups of wild horses."
Gawne says the muster will cost nearly $US2 million, plus another $US1 million for sorting and transporting the horses.
"Are they crazy in this economy? The Calico range is not in poor condition and the horses are healthy, but the BLM increased livestock grazing permits significantly last year. That's the core issue."
Gawne says: "Every year I go on safari to Africa's wildlife parks where wildlife tourists spend millions and employ thousands of local people. Many tourists have asked me where they could view wild horses in North America? My answer is nowhere!"
"Those would-be tourists are shocked and disgusted when I explain that the BLM holds 34,000 horses in pens at a taxpayer cost of over $US100,000 per day, yet sources independent from the BLM estimate there may be just 15,000 mustangs left in the wild."