The U.S. District Court has blocked construction of major segments of a bicycle path along Little Falls Branch Park in Bethesda, at least until the effect that the trail might have on the park and surrounding neighborhoods is reviewed by local planning agencies.

However, the court has permitted construction to proceed in the southern part of the park south of Massachusetts Avenue, where Montgomery County is proposing to build a new underground sewer line along the streambed with a bike trail over it.

That section is now woodland. Little Falls Branch Park extends between Bradley and MacArthur boulevards. A parkway was planned to run the entire length of the park, but in the mid-1960s county officials decided not to extend the parkway south beyond Massachusetts Avenue.

The suit appears to be a setback for the residents who went to court last spring to stop the sewer and the part of the bike trail south of Massachusetts Avenue. Judge Oliver Gasch ruled that section of bike trail, about a mile long, and the sewer replacement line had been given "the necessary hard look" by local agencies and were "reasonable decisions."

The citizens, who formed Save the Westmoreland Woods Inc. to fight the projects, have questioned the necessity of replacing the present sewer and have opposed construction of a bike trail because they felt it would intrude - and possibly bring crime - into what is now "primal" woodland behind their homes.

While Gasch approved the sewer and trail in Westmoreland Woods, he enjoined construction of an other portions of the proposed bike path "until a more thorough assessment" is made of its impact.

The Maryland-National Capital Park and Planning Commission has approved a hiker-biker trail through the entire park between Bradley and MacArthur boulevards that would be more than three miles long. Construction was to begin within the next few weeks on the major section from Bradley to River Road, in conjunction with a parkway repaving project. Bike trails along the other sections had not been fully planned or funded.

Gasch said that since no specific route for the rest of the trail had been designated, it was impossible for officials to determine how many trees would have to be felled or how much of an environmental impact the entire trail would have. He also said county officials had not studied other alternatives, such as putting the trail along the nearby B&O Railroad tracks, which the Save the Westmoreland Woods group had said was soon to be abandoned by the railroad, nor had they considered "the potential for increased crime . . . as a result of the bike path."

A relatively brief environmental review, concerned mainly with the bike trail and sewer reconstruction projects south of Massachusetts Avenue, was done in June by the National Capital Planning Commission.

The federal planning agency for the Washington area, NCPC is involved because the park - and other stream valley parks like Rock Creek - were acquired in part with federal funds and there is still considered to be a federal interest in them. Similar bike trails, paved eight-foot-wide paths with grassy shoulders, have been built in Rock Creek, North Branch and other stream valley parks, but no environmental reviews have been done on them because NCPC and county officials all considered the path to have minimal impact on the parkland.