By 2:45 p.m. the group of 20 was waiting nervously on N Street NW for the pony to arrive on this memorable prelude to the official opening of the Old Post Office Building.

"We've had some problems," fretted Leila Smith, an early crusader for the preservation of the Old Post Office. "Up until last week, we couldn't get a permit for the pony."

Problem One: You can't park a pony on N Street.

Anyway, the idea was to instill a sense of history--and perhaps a little hype--for those involved opening day ceremonies, set for noon today.

A Pony Express rider was to trot up to this self-congratulatory luncheon yesterday for some of the original supporters of the project and deliver invitations to a bevy of events.

And then it came . . . waddling up N Street.

"Where have you ridden from?" called out one of the guests at the Iron Gate Inn.

From Vermont, the beginning of the Pony Express excursion for the spectacular opening day?

From New York, where more invitations were dropped off?

From Philadelphia, perhaps? Or maybe the White House, where Ronald Reagan got his invitation earlier yesterday?

"Oh, I just came from around the corner," said rider Kay Gordon. "We have a stable at the American Forestry building."

So much for history.

But everyone eagerly swooped up their invitations from Gordon. Including Alison Owings. The last time Owings saw the Old Post Office Building was in 1971, when she was standing in front of it with a bullhorn shouting, "Don't Tear It Down." And, of course, as everyone knows by now, they didn't.

Yesterday Owings, an original fighter for the historic building, was one of the star attractions at celebrations all over town for the gala opening of the renovated granite structure, which appropriately sits on Pennsylvania Avenue, midway between the Capitol and the White House. It was Owings who founded "Don't Tear It Down," the preservation group which in the end saved the building.

Recently renamed the Nancy Hanks Center (which includes a performing arts center, retail stores and office space) in honor of the late chairman of the National Endowment for the Arts, the building was singing and dancing last night. The next few days will bring parades and parties and a gaggle of receptions, including last night's black-tie gala, where the National Symphony Orchestraplayed, the Washington Ballet danced and the cast of "Sophisticated Ladies" performed.

Twelve years and more than $20 million later, Owings is loving it.

"I never imagined that I would have a part in saving anything so big," she said. "I was originally aiming to save a bunch of row houses at GW George Washington University . . . It really is sort of toney, isn't it? I'd like the old Willard Hotel to be turned into low- or middle-income housing. I'd like to see some kind of balance."

The Old Post Office has been threatened with demolition for much of its life. It opened in 1899 and was rejected by the Postal Service only 12 years later. Since then, life for the building has been a series of appeals to keep it off death row. By 1977, the General Services Administration was committed to restoring the building.

Postmaster General William Bolger was clearly one center of attention at the late-night party. This was perplexing since he has absolutely nothing to do with the building in spite of it's name. But this didn't stop the flashing cameras and scribbling press.

"No, we don't want it back at all," he said repeatedly to the same question. "We haven't had it for 50 years."

This seemed to satisfy the nosey reporters and they moved on.

Proceeds from the $75-a-head dinner will go to the Cultural Alliance of Washington, a trade association and lobbying group for artists. An estimated $20,000 was raised.

"This is fantabulous," announced Mayor Marion Barry. "That's a combination between fantastic and fabulous."

Robert Irwin wasn't about to take any chances when the GSA contacted him to do a sculpture for the atrium of the vast building. "I must have made about four trips here and sat inside the building for more than 100 hours just looking," said Irwin, a California artist, who ultimately got the $80,000 contract.

Yesterday was also the unveiling of his work: 48 nylon panels suspended seven stories from the sun roof of the atrium.

"It was a very critical assignment because I was working in someone else's backyard," he said at a reception in his honor. "They have to like it."

They did.

Frank Hodsell, chairman of the National Endowment for the Arts, called it "extraordinary," and everyone applauded.

"After two weeks of 17-hour days," said Irwin, "it's a relief. At this point in my career, I thought I would be a gentleman painting paintings in a sunlit room, sipping brandy. Instead, I feel like a plaster contractor."

Owings, who moved to New York shortly after starting the campaign to save the Old Post Office, was like a kid at her own birthday party yesterday, moving from group to group, the center of attention wherever she landed.

"Things just moved and the ball started rolling," she said at the Iron Gate Inn luncheon, "and thanks to everyone here, it wasn't a wrecker's ball."