More than 150 people crowded the sidewalk in front of Senator Feinstein's office in San Francisco to ask for his help in stopping the ongoing Nevada roundup and removal of mustangs.
The advocates want a moratorium on all wild horse musters pending development of a more sustainable long-term strategy.
The eight-week muster, in an area known as the Calico complex, aims to remove up to 2700 horses.
The biggest of the protests was in San Francisco, where more than 150 people crowded the sidewalk in front of horse-friendly Senator Feinstein's office to seek his help in stopping the roundup.
The crowd was joined by Figaro, a miniature horse, and Fluffy, a miniature donkey.
Others were held in Chicago, Illinois; Colarado and Idaho. Others are planned for next week in Las Vegas, Los Angeles and Denver.
Ginger Kathrens, executive director of the Cloud Foundation, which helped organise the protests, labelled the Calico muster as an unnecessary and cruel winter roundup.
"It insults the democratic process if Bureau of Land Management gets away with this. They have betrayed the trust of the American public," she said.
Ecologist Craig Downer called the muster "a devious ploy to displace the wild horses from their legal herd areas. It is very vicious and must be exposed and stopped."
In Defense of Animals president Elliot Katz said: "The BLM knows that its latest assault on wild horses is so shameful that it cannot stand the light of day."
Neda DeMayo, founder of the Return to Freedom American Wild Horse Sanctuary and a member of the American Wild Horse Preservation Campaign, said a lot of US tax dollars were being spent on the removal of wild horses while less intrusive and less costly 'in the wild' management alternatives were available.
"Americans want their wild horses to remain free and protected on the ranges where they currently exist," he said. "We are asking that the original spirit and intent of the Wild Free Roaming horse and Burro Act be upheld so that our grandchildren's children will know the herds, who are a vital link to our western heritage."
The bureau intends to remove up to 12,000 horses from their western ranges each year for the next three years. Most will join the 33,000 wild horses already being held by the bureau.