"It will be the secret of the century," promised Elizabeth Emanuel when the announcement was made that she and her husband, David, would design the wedding dress of the century for Lady Diana Spencer.

And so it has been.

"I can't wait till I finally raise my blinds," David Emanuel said yesterday.

They've been drawn at the Emanuels' Brook Street salon and atelier to conceal the work on what will be the most photographed and most copied dress since, well, Cleopatra's sheath.

"From the beginning, people were peering in with zoom lenses," said Cindi Watson, in-house publicity person for the couple.

Had the Emanuels lifted the blinds yesterday, they would have seen a crowd of reporters and photographers blocking traffic on the narrow street. Word was out that Lady Diana was due in for a fitting, and, indeed, she showed up.

"There was so much commotion the police came," said Watson. But the crowd got hardly a glimpse of the bride-to-be, her entrance and exit were so quick.

The Emanuels have taken special precautions to keep their designs for the bride and bridesmaids secret.

First, they carefully demolished the original sketch shown to Lady Diana and never made another one. Since the discovered "people going through our dustbins," they have carted away their trash and disposed of it secretly. As an extra safeguard, they have discarded many different color fabric swatches all at once so that nolone can determine the color scheme they are using. In fact, they ordered extra fabrics in several color waves so that even the fabric houses wouldn't have a clue about what is being used.

From the start, David Emaneul scaled down the number of people who would work on the wedding dress. "There will be just too much pressure on those working on the dress, and it won't be fair," he said. In addition, he has divided up the sewing assignments like a jigsaw puzzle so that the seamstresses can honestly say they don't know what the whole dress looks like, only the part they are working on.

All of this has not kept people from trying. Bribes have been offered for a glimpse of the dress and dismissed. The Emanuels have denied rumors that they demanded payment for interviews.

Even Women's Wear Daily, the trade daily which once instructed a reporter to go over the wall of the White House to get the advance sketch of Lynda Bird Johnson's wedding gown, has been foiled. Pouting in print, WWD has taken to calling Lady Diana "Lady Dimples" and referring to the Emaneuls' style of design for wedding dresses as "Wagnerian splendor." Louise J. Esterhazy (a cover name for publisher John Fairchild) wrote in a recent issue, "Good luck, Di, though I'm sure your bouyant youth and flashing dimples will help lighten things up."

The Buckingham Palace press office will release a sketch of the dress following the wedding, and the Emanuels will hold a press conference later in the day, organized by a marketing firm that expects to merchandise their names into household words. An Emanuel perfume is scheduled first.

But even if someone got to see the dress this minute, it would not be the final product.

"There is still something to be done," said Watson. "It is a tradition for good luck not to finish the dress until just before the wedding."

And just what part wasn't finished?

"We're not saying anything," she replied firmly.