The U. S. Postal Service, in a decision infuriating to Montgomery County officials and a number of Potomac residents, has approved construction of a large post office at the main intersection in Potomac Village, an agency spokesman announced yesterday.

Frank P. Brennan Jr., a spokesman in the Postal Service's Philadelphia regional office, said the planned $2.6 million, 17,000 square-foot facility just west of the intersection of River and Falls roads will meet the needs of merchants in Potomac Village, an affluent community now served by a one-window post office.

Some Potomac residents, who have fought the larger facility for nearly 15 years, and Montgomery County officials yesterday attacked the planned post office as an environmental and safety hazard. They also said the larger facility would create massive traffic tie-ups at the already overloaded intersection.

Norman Christeller, chairman of the county planning board, which voted to oppose the new post office last year, said he may go to court to prevent construction on the 2.9-acre site near Rock Run creek.

Christeller said the planned post office would violate the county's 1980 master development plan, create traffic jams at the River-Falls Road intersection and threaten the quality of Rock Run. A portion of the post office site is in the creek's flood plain, he added.

Allan S. Cohen, a Potomac resident and past president of the 500-member West Montgomery County Citizens Association, which also opposes the project, said he was "shocked" by the decision to proceed with construction. Cohen and other area residents started meeting with postal officials in March, when the agency announced it would delay a decision until local residents reached "consensus" on the post office location.

"No consensus was ever reached since that time," said Cohen, who with others favored construction of a "split facility" to divide mail sorting and customer services into two small offices in the area. "They pulled an end-run on us," he said.

Brennan claimed a "silent majority" of the 47,000 residents the post office would serve want the post office constructed in Potomac Village. He cited a 600-signature petition, which did not specify a construction site, as well as an endorsement from the Potomac Chamber of Commerce.

Eleanor Pisarra, a Potomac Village boutique owner and president of the 108-member Chamber of Commerce, hailed the planned post office as a boon for local businesses.

Merchants and others in the 20854 ZIP code area must travel several miles to the Twinbrook post office in Rockville to pick up certified mail or large packages, Pisarra said. Construction of the larger post office, she added, was endorsed by a majority of the 18 directors of the chamber of commerce, not the entire membership.

While offering no specifics, Brennan said the Postal Service will take the necessary steps to ease traffic and safety problems and the agency has a plan for construction outside the flood plain.

The new post office would employ 60 people and have 100 parking spaces, 35 postal jeeps and 1,000 lock boxes, said postal officials, who estimated the purchase price for the small site at $800,000. Construction would start in 1985.