The U.S. Postal Service, in a decision that one top Montgomery County official described as "thumbing its nose" at the county planning process, has purchased for nearly $1 million a controversial site near the main intersection in Potomac Village to build a large post office.

Frank P. Brennan Jr., a spokesman in the Postal Service's Philadelphia regional office, said this week that the agency had purchased for $1,050,000 a 2.9-acre tract just west of the intersection of River and Falls roads.

The planned $2.6 million, 17,000-square-foot facility, which would employ about 60 people and have 100 parking spaces, 42 postal jeeps and 1,000 lock boxes, should be ready by 1987, postal officials said.

Announcement of the purchase of the property, known as the Moran site, prompted vehement criticism from a number of Potomac residents, who have fought the facility for nearly 17 years, and Montgomery County officials who consider the planned post office an environmental and safety hazard.

"I have, for several years now, been wrestling with an 800-pound gorilla," said Norman L. Christeller, chairman of the county planning board. "There is nobody who has any control over the Postal Service. They're running contrary to our land-use policies and, in my opinion, wasting public money to do it.

"The Postal Service is thumbing its nose at the planning authorities," said Christeller, adding that planners are considering filing suit against the Postal Service to halt the planned facility.

Planners and citizens groups in the area have charged that a postal facility built on the Moran site would create more congestion at the main intersection, ruin the environmental balance of the adjacent Rock Run stream, which empties into the Potomac, endanger schoolchildren walking from nearby Potomac Elementary School, and be inconvenient to the residential center of the 20854 ZIP Code area, which lies further north and east of Potomac Village.

"The Postal Service has completely ignored the wishes of the community and is completely immune, it seems, to responding to any community needs and interests," said Fernando Bren, president of the West Montgomery County Citizens Association. He said that the association also plans to consider taking legal action against the Postal Service.

Besides the county planning board, other government agencies such as the National Capital Planning Commission, the Montgomery County Council and the Metropolitan Washington Council of Governments have stated opposition to locating the facility on the Moran site. While postal officials said that the agency attempts to adhere to local land-use planning policies, it is not bound by any federal, state or local laws to do so.

Brennan said that the Postal Service plans to take the necessary steps to ease safety and traffic problems, such as installing a right-hand-turn lane off River Road into the site, and that the agency has a plan for construction outside the flood plain.

He said that in the past several years the agency studied up to 24 different sites throughout Potomac Village and determined, as did an independent environmental consultant, that the Moran site would best meet the needs of the Postal Service and about 60,000 customers. To appease citizen concerns, the Postal Service plans to seek input from area neighborhood groups on the design of the post office, Brennan added.

The controversy surrounding the planned postal facility isn't the first time that Potomac residents have waged war with the Postal Service: A few years ago, residents of the affluent Kentsdale neighborhood were unsuccessful in their attempt to block construction of a postal training center in their community.

Potomac is now served out of a split postal facility, which local merchants and some residents have complained about. Counter service is handled out of a closet-size contract operation in the Potomac Village Pharmacy, while special pick-up mail and carrier route sorting are done several miles away at the Twinbrook post office in Rockville