After 24 hours of bargaining that involved the Washington area's top elected officials, the Metro board this morning bowed to heavy pressure from the District of Columbia and agreed that work should begin on part of the subway's mideity Green Line next year.

The agreement came just when it looked as if members of Metro's board were going to shoot themselves in the feet and halt a $323-million construction program that needs the District government's blessing if it is to get under way in the current fiscal year, which ends June 30.

"This is a very big step forward," Metro General Manager Richard S. Page said. "Yesterday, I despaired that we were not going to be able to do it."

The agreement guarantees only that construction on the Green Line will have equal priority with other Metro projects when the 1982 construction program is negotiated in months to come. It is clear, however, that the District will settle for nothing less than some work on the Green Line that year.

D.C. Mayor Marion Barry said Thursday night that he has personally settled two previously unresolved issues regarding where the Green Line should run in the District, and that he intends to move quickly to resolve the others.

Page said that those two decisions, plus work already under way or scheduled, would make it possible for Metro to operate a short Green Line from Anacostia to U Street through the heart of the city's most disadvantaged areas while other sections were under construction.

Nobody knows how much money the federal government will be willing to contribute to Metro construction in the next few years, and that determines how quickly things can be done.A U Street-Anacostia Green Line could be running in 1987, according to one back-of-the-envelope Metro estimate.

Barry's decisions clear the way for the resumption of design work on two stations -- the Shaw Station, under 7th Street between R and S streets NW, and the U Street Station, under U Street between 10th and 13th streets NW. Design has been in limbo for several years because of discussions within the District government about moving the Green Line farther north and closer to Howard University. Barry said last night that too much time would be lost if such a major redesign were done.

Part of the Green Line tunnel and station work has been completed, including the Waterfront Station in Southwest, and a connecting tunnel under 7th Street that runs through the L'Enfant Plaza, Archives and Gallery Place stations. L'Enfant Plaza and Gallery Place already are open and serving other lines. Metro already has scheduled work from the Waterfront through the Navy Yard to the Anacostia station, and that section is scheduled to go into operation in July 1986.

The District surprised the Metro board yesterday by demanding that Metro "advance the construction scheduling [on the Green Line] . . . to the earliest possible date . . . . " If the suburban partners were not willing, the District said, it would withhold $189 million in federal money it controls. That would have had the effect of halting all of Metro's 1981 construction.

A bargain was reached quickly, partly because all the key players, including D.C. Mayor Marion Barry, Montgomery County Executive Charles Gilchrist, Prince George's County Executive Lawrence Hogan and Fairfax County Board Chairman John F. Herrity were here -- along with Metro board members from all jurisdictions and more than 100 other area officials -- for the annual briefing on Metro's progress and budget.

Page and his lieutenants shuttled from meeting to meeting and negotiated into the night. Hogan and Gilchrist had an important meeting early this morning to resolve Maryland issues. The final language of the compromise was agreed upon in one corner of a meeting room here just before the board formally met.

Maryland and Virginia officials were concerned that advancing the Green Line would delay projects in the states. However, there was acceptance from the beginning that the District had the upper hand; without District money, suburban construction would suffer. Furthermore, such tactics are not unprecedented -- Montgomery County ran a similar successful power play three years ago to protect the Glenmont Line.

Finally, the District was willing to settle for something less than a written guarantee that the Green Line would be first. The language of the agreement calls for the "start of construction of the D.C. portion of the [Green Line] as a concurrent priority with other high-priority projects . . ."

That is a classic Metro compromise. The District releases the money that keeps the construction program moving in the suburbs and wins acceptance of its new interest in the Green Line. But the construction schedule for fiscal 1982 -- and the contract and funding necessary for that construction -- are left for another day.