As auctioneer David Fox cried "I've got nine million, five hundred thousand dollars once . . . " and a tentful of speculators and spectators held their breath, the president of the National Food Processors Association yesterday bid a record $530 a square foot for a half block of downtown Washington real estate.

Outbidding three of Washington's biggest land developers at an unusual public auction, the trade association agreed to pay at least $100 a squre foot more than anyone has ever paid before for a downtown property.

Association President Charles Carey said the food processors' group plans to build a new 12-story headquarters on what is now a parking lot on the east side of 14th Street NW between New York Avenue and H. Street.

The $9.5 million price will produce a profit of more than $8 million for the property's owners -- a group headed by Harold Zirkin and Jon Gerstenfeld -- who bought the land for $1.2 million in January 1977.

The 14th Street corridor was considered too sleazy a neighborhood for prime development 3 1/2 years ago, but yesterday, some of the same investors who turned down the property at $1.2 million showed up to bid in an auction where the opening bid had to be at least $7 million.

Although there are homosexual bathhouses, pornography parlors and coin-operated skin-flicks across the street, promoters of the auction stressed that the site was "only two blocks from the White House."

The former site of the Trans-Lux Theatre is also the biggest tract of land avialable for development in downtown Washington, said William Fox, who with his brother David run Michael Fox Auctioneers of Baltimore.

Doing business out of a custom-built motorhome, the Foxes auctioned off the property beneath a yellow and white stripped tent carpeted with artificial turf. There were reserved seats for bidders who had to put up $250,000 just to get a number allowing them to make a bid.

Eleven people paid the ante, and more than 200 others showed up to watch the auction of a piece of downtown property in years.

The sellers decided to auction off the land "because they believed it would maximize the interest in the sale and maximize the price," explained their attorney, Earl Colson of Arent, Fox Kintner, Plotkin & Kahn.

The minimum bid had to be $7 million -- otherwise the owners would keep the tract -- and auctioneer David Fox began his spiel by "inviting" a bid of $9 million.

He didn't get it, and for a moment it looked as if speculation that no one would meet the minimum price might prove correct.

Then a young man in sunglasses flashed a card with the number 201 and nodded a $7 million opening. He turned out to be Evan Novenstein, a lawyer representing Theodore Lerner, developer of Tysons Corner, White Flint and other projects.

As the bidding jumped $100,000 at a pop, the roomful of Realtors strained to see where the action was coming from, and the whispered names of the players swept through the tent.

When the bidding seemed to be stalling at $9 million, auctioneer Fox began extolling the virtues of the vacant lot: "It's still an extremely cheap price for what you're getting," he purred, and the crowd cracked up.

A half-million dollars later, Fox paused a moment, pleaded for a final bid, dropped his hammer and lead Carey into the motorhome to sign the deal.

Terms of the sale require the successful bidder to pay an additional 3 percent of the auction price as a "buyer's premium." That adds $285,000 to make the total $9,785,000 -- or $530 a square foot for about 18,460 square feet of land.

The highest previously known price for a piece of downtown property is the $400-a-square-foot that the John Akridge Co. agreed to pay last year for the YWCA at 17th and K Streets NW.