Washington's historic Old Post Office Building unveiled its boutiqued new interior yesterday, with a rain-drenched grand opening marked by damp but jubilant crowds and enough ballyhoo and hoopla to make P.T. Barnum blush.

Two brass bands, 6,000 balloons, Mayor Marion Barry, and four marching overnight letters celebrating Federal Express all trumpeted the long-awaited arrival of shopping mall culture in downtown Washington.

"This is a significant moment in the history of this city," said Charles Evans, partner with the federal government in the $38 million renovation of the towering Romanesque structure at 12th Street and Pennsylvania Avenue NW. "What we've created is hard to describe."

Inside, dwarfed by the building's flag-hung, seven-story atrium, Diana Williams, 21, agreed. She was dressed as a giant strawberry, a stem growing out of her head, and she paced the marble-tiled corridors celebrating an eatery called "Flying Fruit Fantasy," nearly lost in a forest of twinkle-lit ficus trees.

Last week she was peddling perfume behind a counter at Woodies. "This is better," she said. "I wanted something different."

The Old Post Office is, indeed, something different. A functional cross between Georgetown Park mall and a Hyatt Regency Hotel, it features 22 specialty shops and 19 restaurants tucked within the granite walls of a building so authentically Medieval its tower boasts slits for crossbowmen firing in defense.

Above the three floors of shops and restaurants, eight floors of federal offices surround a vast atrium roofed by a skylight two-thirds the size of a football field.

A glass elevator, scheduled to make its first trip next spring, will carry tourists to an observation platform high within the bell tower, second tallest structure in the nation's capital.

Yesterday's grand opening, however, was for the Pavilion, the $10 million commercial complex developed by Evans and touted by a succession of speakers as the catalyst for a social, cultural and economic Renaissance downtown.

"What we're talking about here is a revitalization and a rebirth . . . ," said Henry A. Berliner, chairman of the Pennsylvania Avenue Development Corporation. "It means that all of a sudden . . . Pennsylvania Avenue becomes a place not only to work but to eat, to shop and hopefully, with the building of residence properties on the avenue, a place to live as well."

To celebrate all that, Charlotte Sykes, director of marketing for the Pavilion, had been working with a staff of seven since June 5 on yesterday's ceremonies. They had been slated to feature a 50-unit parade, Pony Express riders, a remote broadcast by WMAL Radio, extended speeches and a fountain of red, white and blue balloons, released by the ribbon-cutting into a sunlit sky.

It didn't work out that way. Fifteen minutes before the scheduled noon opening, the rain was descending in buckets and the parade had dwindled to a few horses, some motorcycle policemen, the Federal Express truck and its accompanying marching envelopes and a damp but undaunted unit from Yorktown High School in Arlington.

Sykes was not flustered. But neither was she happy.

"I've got to get this band to play something--ANYthing," she said, striding with her crackling walkie-talkie to the Largo High School Band, its members huddling beneath the portico out of the wet.

But even that didn't work.

"He said he only brought two songs," she said, returning. "And one of those is the Star-Spangled Banner."

By the time Barry arrived, resplendent in a Redskins Stetson, the ceremonies were dwindling fast.

Three stories above the rain-soaked speaker's dais, Debbie McAteer, 32, stood poised on the Post Office flag balcony, together with a handful of soggy volunteers from the National Endowment for the Humanities.

Their mission was to loose upward several hundred of the day's 6,000 red, white and blue helium-filled balloons upon the ribbon-cutting signal from the troops below.

The Endowment volunteers, through whose office windows McAteer had clambered with her balloons minutes before, had been given a half-day's leave to join the festivities, and let go their charges right on schedule.

But instead of the balloons going up, the rain came down, soaking the balloons and strings and impeding their ascent. Inflated bouquets lay huddled in the roof corners as their keepers ran for cover.

But if the rain came, so did the crowds. They jammed the stairways and packed the restaurants and gawked at the open space. They wolfed down sweets from Cone E. Island Scoops and the Cookie Connection and generally pronounced the place grand.

The ultimate perspective on the Old Post Office Building, however, may have come from Mary Gaskell, 42, of Papillion, Neb., who about 1:30 p.m. was wincing over a margarita at Blossoms cafe.

The building was awfully crowded, she said, but she'd be back for a second look when things slowed down. On the whole she liked it: "It reminds me of the restored old market area. In downtown Omaha." CAPTION: Pictures 1 through 3, Mayor Marion Barry, in Redskins Stetson, arrives at opening of $10 million Pavilion; Flags were out in force during ceremonies at the Old Post Office building yesterday; Despite heavy rain, the opening at the Old Post Office Building Pavilion was jammed. People generally pronounced the place grand. Photos by Joel Richardson--The Washington Post