A federal appeals court on Tuesday cleared the way for the roundup of more than 2,000 wild horses in California and Nevada, rejecting critics' claims that the free-roaming mustangs have a legal right to remain on the range.

In an after-hours order, the 9th Circuit Court of Appeals in San Francisco refused to grant an emergency stay sought by animal rights groups ahead of the scheduled roundup Wednesday.

The order came as lawyers for the Obama administration mounted their most vigorous defense to date for rounding up wild horses in the West, arguing in court filings that leaving the overpopulated herds on public rangeland would do the mustangs more harm than good.

Assistant U.S. Attorney Ignacia Moreno said in a court filing that the range cannot support the current population of horses, whose numbers were expected to grow. "The growing population of wild horses will have devastating impacts on the other resources in the area and on the wild horses themselves," Moreno wrote.

Justice Department lawyers were defending the roundup on behalf of the Bureau of Land Management and its plans to begin its biggest roundup of the year along the California-Nevada border, about 120 miles north of Reno. That number represents up to 80 percent of the existing herd.