THE bicycle once seemed the environmentalist's dream machine, a solution to air pollution, traffic jams and flabby muscles. But that was before bicycle makers here in Marin County added gears to some old fat-tire Schwinns and launched the latest rage to roll out of California: the mountain bike, a machine capable of climbing trails and scaring hikers as it roars like a roller coaster in a cloud of dust.

With the number of leg-powered mountain bikes doubling every year, complaints by hikers and equestrians are rising. In recent months, the bulky bikes - souped-up versions of the old heavy-framed models that were popular before the advent of the European 10-speed - have been banned by Federal, state and local authorities from many trails in Marin County, in the Santa Monica Mountains south of San Francisco, on Santa Catalina Island near Los Angeles, in Boulder, Colo., and elsewhere. In other areas, the bikes have been restricted to fire roads and placed under speed limits of 5 to 15 miles an hour.

''There's been damage to property and annoyance to people hiking and horseback riding,'' said Fran Brigmann of Marin County's Parks and Open Space Department. Bikes also contribute to erosion.

The issue has torn environmental groups. The Sierra Club, for example, finds itself divided between members who hate mountain bikes and those who own them. According to Sally M. Reid, vice president, the club is revising its policy of classifying the bikes along with motorcycles as intruders that should be banned from public lands. Instead, they have created a new category of nonmotorized vehicles, to be treated more leniently.