DENVER — Mountain bikers, now barred from most backcountry areas in national parks, could have thousands of miles of trails opened up to them under a rule change proposed Thursday by the Interior Department.

The proposal raised tensions between hikers and bikers, who face off against one another on dirt byways all over the country. Each group is burdened with a stereotype that is part true and part myth: thrill-seeking gear heads on one side, plodding leaf peepers on the other, each group accusing the other of not fully appreciating the great out-of-doors.

"The question is whether it can be managed well — whether one group doesn't deprive others of their enjoyment," said Jeff Ruch, the executive director of Public Employees for Environmental Responsibility, a nonprofit alliance of local, state and federal scientists, law enforcement officers and land managers.

In any case, Ruch added, "it's a symptom of growing user conflict in the national park system."

The proposal would not take effect until the middle of next year at the earliest, National Park Service officials said. Under the plan, many trail usage decisions would be made at the level of individual parks, rather than at the central National Park Service office.

Both sides say that would accelerate trail decisions and probably result in a new arena of discussion — or conflict — at each park headquarters, with administrators being lobbied by the two groups. Existing trails would be the main focus of the change; most proposals for new trails would still have to go through lengthy review.

Opponents said that the rule change could open up to bicycling millions of acres now designated as potential wilderness.

"Seventy-five million Americans hike, and they want solitude and a slow-paced connection with nature," said Gregory Miller, the president of the American Hiking Society, an umbrella group of 275 local organizations. Biking groups say that the national parks are in trouble, and that young people are more likely to connect with outdoor life and physical fitness through biking. Overnight backcountry camping stays fell by about 21 percent from 1997 to 2007, according to the National Park Service.

"We think mountain biking could bring new and younger visitors to the parks who are not now finding the recreational opportunities that they are seeking," said Mark Eller, a spokesman for the International Mountain Bicycling Association, a group based in Boulder, Colo., that has been pushing for a park expansion of mountain biking for years.