When the beautiful peal of bells rang out for the first time last week from the tower of the Old Post Office building, people walking purposefully along Pennsylvania Avenue looked up, surprised. For a moment their glazed, workaday expressions were transformed into something brighter.

It was only a test for Tuesday's official dedication of the restored structure, but it put the sound of celebration in the air. Nothing could be more fitting. The rejuvenation of the massive old building is an accomplishment of great importance and greater promise. This should put a smile on the city's face.

The Old Post Office building, which will open up in stages this year, is not the same old office building that Washingtonians have known for 84 years--it's better. With a viewing platform in the clock-and-bell tower and an alluring assortment of restaurants and shops in its spacious atrium, it is the key piece in the puzzle of downtown renewal--Washington's equivalent of Baltimore's Harborplace or Boston's Faneuil Hall.

And its importance is more than local. As the flagship enterprise of the Cooperative Use Act of 1976, which permits mixed uses in federal buildings, the Old Post Office could signify a giant step ahead in U.S. government architecture.

Simply saving the building, completed in 1899, was the first major step, and before the bells peal in earnest it is useful to recall that this was by no means a simple matter. It took luck and skill in huge quantities.

Luck, in this case, took the peculiar form of great events (the Depression, World War II) that intervened to cut short the completion of the Federal Triangle in the classic revival style, from which W.J. Edbrooke's Romanesque revival building deviated, in the eyes of its detractors, like a sore thumb. When the federal bureaucracy, fortunately acting with a sluggishness outstanding even for it, proposed snipping off this offending architectural thumb to finish the Triangle in the late 1960s--some three decades too late--skill and spunk and vision took over.

The determination and resourcefulness with which the preservationists of Don't Tear It Down fought this unhappy proposition cannot be overpraised. Nor can the imaginative enterprise of architects such as John Wiebenson and Arthur Cotton Moore, who volunteered their vision more than a decade ago to demonstrate the building's rich possibilities. With words and images, the architects argued that the building could become at once a symbol and a generator of diversity in the very place where the city needs it most.

Amazingly, this is precisely the way the story of the Old Post Office building is turning out. Tuesday's "rededication" of the building marks the moving in of the first contingent of the federal work force (mainly employes of the National Endowments of the Arts and Humanities) and the installation of the tower bells, a bicentennial gift from the Ditchley Foundation of Great Britain. It is but the first event to commemorate this significant achievement.

Another ceremony will take place Sept. 1 when the doors open to the Pavilion at the Old Post Office, a complex of 29 "specialty impulse retail" shops and 21 restaurants (five "major" places and 16 "specialty food units"). Early next year a glass-encased elevator in the building's towering atrium will take its first batch of passengers to the tower viewing platform--a spectacular addition to the city's long list of tourist attractions. (Walk-up tours will begin earlier, perhaps as soon as this summer.)

And a time has yet to be selected to dedicate the Nancy Hanks Center. This is the name that Congress, in its wisdom, decided to give to the complex--the building, its entrance plaza on Pennsylvania Avenue and the performing stage in its atrium--in honor of the late former chairman of the National Endowment for the Arts, whose enthusiasm for the dream did much to make it come true.

The superfluity of names--to be absolutely correct, I suppose you'd have to tell a taxi driver, "Take me to the Pavilion at the Old Post Office at the Nancy Hanks Center"--adds an unfortunate, if somewhat comic, note to the whole affair. It will be interesting to see, over the next decade or so, if we can settle upon one of its three names. (Maybe the hackers will have the last word on that.) But the number of ceremonies is perhaps justified: There's a whole lot to celebrate.

It is hard to figure out just what Edbrooke had in mind when he designed his rectangular doughnut, with office corridors surrounding that vast skylit courtyard. Arthur Cotton Moore, whose firm deservedly won the architectural competition to restore the building, points out that it never really worked. The only use Edbrooke could think of for the ground floor of the building was as a mail sorting center, which he covered with glass that always looks filthy in old photographs. As a result, only the office workers ever got a view of the soaring "courtyard" space.

Moore's solution--to open the space by breaking through to the basement level and filling it with things to attract people--allows the building to reach its full potential for the first time. His design for the interior space is appropriately theatrical and subtly respectful at the same time: the three-level shopping arcade steps back in a sequence of streamlined curves ("like tiers in an opera house," he says) from the tower, which stands tall front and center the way it always should have but never did. The tower's base, excavated to reveal its impressive granite footings, forms a fitting backdrop for a semi-circular stage.

The design evolved from Moore's awareness that "this building should play a strategic role in the city." By conceiving of it as a center for day and night activities (his initial, volunteer proposal had been to make the building a hotel) Moore took maximum advantage of its location and its architectural character. The back of the building, accessible from 12th Street, is in the process of being transformed from an unsightly loading dock to an alluring entrance way halfway between Pennsylvania Avenue and the Mall. Another entrance is at the northeast corner of the building, near Pennsylvania Avenue. Moore's strategic thinking basically expands the building's mission: It will become a bridge between the Mall and the city, the first major step in this direction since the feds began clearing the land for their neo-classical office buildings 45 years ago.

Moore's excellent work is being completed in fine fettle by Benjamin Thompson and Associates of Cambridge, Mass., the firm in charge of designing the retail space, and the Evans Development Co. Charles Evans, its president, is predictably and persuasively bullish on the 60,000-square-foot Pavilion project.

"Some people say we're coming in a couple of years early,, but I don't feel that way at all," he says. "Just because you're first doesn't mean you're early."

Evans rattles off impressive statistics to prove the market exists--20 million tourists on the Mall, 115,000 office workers within four blocks of the building, a $20,000 average yearly income for both groups, and so on.

"It's like Baltimore before Harborplace," he says. "There's a large market and no product. Haborplace has the water. That's the attraction. Here, we have a location that's just as good, and a subway stop Federal Triangle right on our doorstep. Our attraction is this great building."