The Metro transit authority acted yesterday to begin digging subway tunnels to link Gallery Place with a new station planned at Mount Vernon Square--a move expected to disrupt traffic on Seventh Street NW for more than two years.

District of Columbia officials said they plan to bar northbound traffic from using Seventh Street NW between I and M streets after the start of excavation work, currently scheduled to begin in about two months.

The southbound lanes of Seventh Street NW would remain open, officials said.

The $25.8 million tunnels are designed to carry Yellow Line trains to their northern terminus, the planned Mount Vernon Square-UDC station at Seventh and M streets NW. The Yellow Line now connects Northern Virginia with the downtown area, running between the National Airport and Gallery Place stations.

The new tunnels would also be traversed by trains on the proposed Green Line, a controversial route planned to extend through Northeast and Southeast Washington to Prince George's County. Construction of major Green Line sections have been delayed by court challenges and neighborhood disputes.

According to Metro officials, subway service to Mount Vernon Square-UDC is currently scheduled to begin in the late 1980s, perhaps as early as 1988.

Metro's board of directors cleared the way for tunnel excavation by awarding a contract yesterday to two construction firms, Mergentime Corp. and Loram Construction, Inc. The board has not yet chosen a firm to build the Mount Vernon Square station.

The two tunnels--one for northbound and the other for southbound trains--will be carved out by workers using a giant hydraulically run device, nicknamed a cookie cutter.

Metro officials said, however, that the huge tunnel-digging machine will be virtually inaudible above ground.

"Nobody will know that it's down there," said Edward Waddell, Metro's construction director. "It's soft earth. There will be no blasting. There should be no vibration."

According to Metro and District officials, the traffic disruption will be caused mainly by two other aspects of the project--the digging of wide shafts in the street to allow the excavation apparatus to be installed underground and the tearing up of pavement to permit removal of utility lines that might block the tunnels' paths.

George Schoene, assistant D.C. transportation director, said northbound drivers who normally use Seventh Street NW will be advised to detour by heading east on I Street and north on Sixth Street NW.

The section of Seventh Street NW scheduled to be closed to northbound traffic was described by D.C. officials as a moderately congested corridor during afternoon rush hours.

In another development, Metro officials announced the arrival yesterday of a new group of articulated buses--extra-long vehicles that bend in the middle.

The first four of these buses, purchased at a cost of $271,877 each from M.A.N. Truck and Bus Corp., a West German firm, have been delivered, and Metro expects to receive another 29 by the end of July.

Officials said they have not yet selected the routes on which the articulated buses will be used. Metro now operates 43 articulated buses on the Benning Road and Connecticut Avenue lines.