Horses from the Twin Peaks Herd Management Area, 25 miles northeast of Susanville, California.
The bureau aims to gather up to 2300 horses and 205 burros from the area. It would retain 1855 excess wild horses and all 205 burros before releasing the remainder.
The released animals will be subject to long-term fertility treatment. The bureau would also ensure 60 per cent of the freed horses were male.
A 2010 census put the number of horses in the 798,000-acre range at 2303 and the number of burros at 282.
The Cloud Foundation said the horses and burros were the bureau's scapegoats for damage on the range, despite being heavily outnumbered by grazing livestock.
The foundation also attacked the proposed timing of the muster during summer temperatures to avoid conflicting with the deer-hunting season. It also questioned the accuracy of the bureau's horse count.
However, the bureau counters that wild horse numbers have increased an average of 20 per cent a year since the area was last targeted in 2006. It said the muster was necessary to protect vegetation, waterways and soil in the range.
Foundation representative Anne Novak, a fifth-generation Californian, said she visited the herd area, outside of Susanville, last week.
"We were looking forward to seeing a lot of wild horses on the range after hearing the bureau's reasons for the proposed roundup but we didn't see any horses at all," she said.
"We found three pronghorn and a small group of burros after covering the range all day on safari."
Deb Coffey, who travelled from Los Angeles to see the horses, said: "We spent all day with the bureau on a scheduled tour, combing the Twin Peaks area looking for wild horses and burros.
"We found a grand total of 20 wild horses, including one foal and no burros. I really don't believe that there are thousands of wild horses in the area."
The foundation is seeking a moratorium on wild-horse musters until long-term solutions are found to their management.
Executive director Ginger Kathrens said it appeared the bureau did not have a handle on wild horse and burro numbers, noting how the bureau came up short in its controversial muster in Nevada's Calico complex earlier this year.
"Twin Peaks is a perfect example of a lack of scientific range data which would include an accurate census. None of the numbers add up and nothing is consistent except bureau's mismanagement of America's treasured wild horses and burros.
"I want to see our California wild horses spared from the nightmare I witnessed Nevada's horses go through during the recent roundup."
Kathrens noted the bureau's Eagle Lake field office, which manages the Twin Peaks herd, reported receiving 2300 letters requesting cancellation of the roundup, but only 15 in support.
Wildlife ecologist Craig Downer, who has been at the centre of legal challenges to the bureau's earlier mustering plans, said the department "prefers to excessively grant land use to damaging livestock rather than honour the wild horse's unique place in the western ecosystem".
He continued: "If the government would consider adaptive and holistic management of America's public land we would all reap the benefits," he said.
"When is the bureau going to bring science to the table and give wild horses and the American taxpayer a break?"