Daniel Patrick Moynihan, the young secretary of the Ad Hoc Committee on Federal Office Space, was at lunch in the elegant Georgetown home of William Walton, the president's great friend, when the call came in from the White House operator that President John F. Kennedy had been shot.

The last instruction Kennedy had given before he left for Texas on Nov. 21, 1963, had been to plan a coffee hour to show Congress the model of his vision for Pennsylvania Avenue, Moynihan said. That planning was under way at lunch the next day when Kennedy was assassinated.

Moynihan, now the senior senator from New York, said "there thus devolved on others" the mission of carrying out the dream. "It has taken a quarter century, but it is now done, or soon will be."

"Now we shall have the International Cultural and Trade Center on 14th Street," he said with yesterday's final passage of his bill authorizing the monumental structure. "It has been worth doing."

Keeping the idea alive after Kennedy's death was the single most difficult task in the 25-year effort, Moynihan said. Today, the avenue "is eclectic, not the same as we envisioned, but I like it fine."

Back in 1961, Moynihan said, as the president rode back from the Capitol in his inaugural parade, he noticed on the south "the Federal Triangle was left unfinished and looking derelict after work just stopped in the Depression."

One end of the building to the east of the planned trade center was bricked up; the granite to match the rest of the facade was never installed.

"On the north was a slum," Moynihan said.

At a Cabinet meeting on Aug. 4, 1961, the ad hoc group told the president "the north side presents a scene of desolation, block after block of decayed 19th century buildings, many of which are vacant above the first story, only rarely interspersed by partially successful efforts at modernization."

The Capitol, the ad hoc committee pointed out, "is increasingly cut off from the most developed part of the city by a blighted area that is unsightly by day and empty by night.

"This city was a different city," Moynihan said in an interview. "Development was floating out Wisconsin and Connecticut avenues and the area around the Capitol was depressed."

Efforts to remake the avenue had been hampered because wooden buildings put up in the last century had "passed down to the eighth generation, with 18 or 19 owners scattered all over. There was no way to get a clear title to a square block," he said.

"Bit by bit and place by place," Moynihan said, the redevelopment projects have made a difference. "They have not been uniformly successful, and they are anything but uniform, but they have saved the avenue."

In June, before the Senate Environment and Public Works Committee, Moynihan offered this description of the redevelopment:

"The fine new Canadian Embassy is rising at the foot of Capitol Hill. Across the avenue, I.M. Pei's masterful East Building seems now to have been there from the beginning . . . . The semi-derelict Patent Office of a quarter-century back is now the vibrant National Portrait Gallery and National Museum of American Art. At 9th Street we come upon the massive FBI building, which J. Edgar Hoover moved back 50 feet to allow for the avenue's plan. Then we come to the magnificently restored Old Post Office . . . . At 14th Street, we come upon glory indeed with the reopened Willard Hotel, in ways the queen of the avenue."

Yesterday, with the House voting to approve a $362 million International Cultural and Trade Center in the last vacant site on the avenue, Moynihan paid tribute to Harry C. McPherson Jr., counsel to President Lyndon B. Johnson, and to former president Richard M. Nixon for keeping the enterprise alive.

"It is especially satisfying that we have ended up not merely saving the center of Washington but saving almost a third of the billion in rent money {the government would otherwise have spent to lease future office space} as well," Moynihan said.

"After a quarter century, I believe our work is done and I respectfully ask to be relieved."