Construction has begun on 127,000 square feet of office space at Seneca Corporate Center on Interstate 270 in Gaithersburg.

The American Computer & Electronics Corp. is developing 60,000 square feet into a $4 million office building. The structure, designed by architect Stanley T. Lewis, will occupy 4.2 acres of the 45-acre park.

The computer company will occupy one-half of the building.

Tektronix Inc., an electronic equipment manufacturer based in Beaverton, Ore., will occupy a 67,000-square-foot structure being built on 14.2 acres. The office building is part of a master plan with a total of 215,000 square feet of space to be constructed over the next five years.

Both buildings are scheduled for completion in 1983.

Recent developments in commercial leasing in the Washington metropolitan area:

* General Electric Information Services Co., the western division of GTE Sylvania Systems Group and the Chamber of Commerce of the United States have leased space in the Commerce Center 270, located in Rockville near Rte. 28 and Interstate 270.

GTE has indicated that it plans to recruit the majority of its workforce for the new center from the Washington area. General Electric, which has its headquarters in Rockville, will locate its program management operations at the center. The Chamber of Commerce will relocate its mid-Atlantic marketing center there.

* Expansion of the Sherman Fairchild Technology Center, owned by Fairchild Space & Electronics Co., began last week. The center is located in Gaithersburg.

* Aerospatiale Aircraft Corp. has leased 3,085 square feet at 1110 Vermont Ave. NW. The company, a subsidiary of Societe Nationale Industrielle Aerospatiale, markets the ATR-42 aircraft, a 42-passenger commuter plane.

* The consulting firm of Reynolds Thomas Management Inc. has leased over 4,600 square feet at East-West Towers in Bethesda.

* Intertec Data Systems Corp. has leased over 4,500 square feet at Montgomery Executive Office Center in Gaithersburg.

* Tab Products Inc., an office products company, has leased over 3,700 square feet at 7945 MacArthur Blvd. in Cabin John.

* Safe Deposit Corp. has leased over 2,000 square feet at Tysons International Office Center in Vienna.

* 1831 Associates has sold a 6,300-square-foot commercial townhouse at 1831 M St. NW. to Daniels Inc., which is owned by Daniel Chreky. The sale price was $775,000.