The U.S. Postal Service began its long-threatened demolition of the 65-year-old Frederick, Md., Post Office to create a parking lot yesterday, just as the U.S. General Accounting Office concluded that the demolition was unnecessary to provide parking and that $307,000 could be saved by letting the structure stand.

Postal officials dismissed the GAO's report late yesterday as "an oversimplification" and announced that while only a small portion of the building was demolished during the day they planned to continue razing it on Monday.

Preservationists argue that replacing the Greek Revival-style building with a parking lot would harm the city's historic district.

Early in the day, city officials refused to give a Baltimore wrecking firm permits to raze the structure or to use heavy equipment on city streets. A notice was then hand-delivered to the wrecking crews, stating they were violating city laws.

However, postal officials told city officials over the telephone they "don't have to comply with local zoning or historic district laws" and planned to ignore them, Carolyn Greiner, executive assistant to Mayor Ronald Young, said late yesterday.

The GAO report found there was sufficient parking already around the new post office, built beside the old building in 1977, and said the postal service's recent offers to sell the old structure were a clear "admission that customer parking needs can be met without demolishing the old building."

Postmaster General William Bolger last fall agreed to sell the building, then changed his mind. He again agreed to sell it last month, shortly after First Lady Nancy Reagan presented a historic preservation award to Mayor Young for "saving" the old post office and other historic buildings in Frederick.

A Frederick businessman agreed to buy the building at its $122,000 appraised price, but postal officials rejected it Thursday as inadequate on other grounds.