Old downtown Washington is back.

The omnipresent cranes are still twirling overhead, hard-hatted workmen pour concrete throughout the day and pedestrians still have to dodge in and out of covered construction walkways.

But suddenly, seemingly overnight, the District's one-time business center, shunned by shoppers for years and ignored by corporations, trade associations and law firms seeking a workaday home, has shed its forlorn visage and taken on a new, restored and decidedly upscale look.

Yesterday the latest gem in the city's revitalized downtown core opened -- Hecht's Metro Center on G Street between 12th and 13th, the $40 million beige marble and granite flagship of the 23-store chain.

With red awnings outside and trendy, but pricey merchandise inside, Hecht's has built the largest free-standing downtown department store in the United States in the last 40 years.

A veritable checklist of development has preceded Hecht's: The National Theatre has been refurbished, the marble-clad flagship of the Marriott hotel chain is a reality, the restoration of the Old Post Office completed, the Shops at National Place filled with 10,000 or more customers a day, the rehabilitation of the Willard Hotel started.

In the last two years, several gleaming new office buildings have opened their doors in the old downtown bounded by 15th and Ninth streets NW and New York and Pennsylvania avenues, such as Metropolitan Square across from the Treasury and the glass-fronted structure at 1201 Pennsylvania Ave. More office and retail monuments to Washington's continued economic vitality are on the way, including the I.M. Pei-designed, pink and gray granite office building on 13th Street being constructed by Houston developer Gerald D. Hines.

It is perhaps with only a hint of civic boosterism that Curtis R. McClinton Jr., the District's deputy mayor for economic development, can claim, "Washington is perceived as the smart money place to be. The positive metamorphosis will continue to occur."

With a supportive Mayor Marion Barry proclaiming, "Washington is business," and local, national and international developers vying to gain a foothold here, the old downtown landscape has been markedly transformed since 1983.

In the last 20 years, the city's traditional downtown, with its collection of turn-of-the-century three-story buildings, became the ugly stepchild to the newer office buildings, often with atriums, along K Street NW and the glitzy shops of Connecticut Avenue.

Developers and city officials point to a variety of reasons for the decline of downtown D.C. -- the advent of the interstate highway system and the proliferation of new suburban shopping malls preferred by many city residents who left the city for nearby Maryland and Virginia, the fears prompted by the 1968 riots after the assassination of the Rev. Martin Luther King Jr. and a pervasive feeling among investors that American center cities were dying.

Now, after years of planning and talk, old downtown is emerging as the new downtown. Government and business officials cite a number of factors for the turnaround. There was the regional investment in the Metro subway easing downtown traffic congestion, the city's construction of the Washington Convention Center, the largely private rehabilitation of Pennsylvania Avenue and its environs under the oversight of the federally sanctioned Pennsylvania Avenue Development Corp., the city's pro-development stance, the lack of more land for development west of 15th Street, and, in recent years, readily available, multimillion-dollar financing.

But officials say that it was not until the last two years that Washingtonians generally had a feel that their city's downtown was again a place to visit, let alone shop or work.

"There comes a point when there is a critical mass" of evidence of change, said Henry A. Berliner, chairman of the Pennsylvania Avenue agency. "It's not just the Marriott, the Old Post Office or the National Theatre. It's the combination of all those.

"What people look for is a total living environment -- the cultural, the entertainment, the stores, the restaurants, the housing," Berliner said. "Then you'll only go to the suburbs for a lark, for a change."

Developer Oliver T. Carr, Washington's kingpin of office construction and an early believer in the possibilities of a revived downtown, said, "The biggest change is psychological. So much has happened that people can see and touch." Carr's company alone has $550 million worth of current or planned projects in old downtown. His company brought Hecht's to its new location.

J. Warren Harris, Hecht's board chairman, said that all of the recent additions to downtown serve as "catalysts to make people want to come back downtown. You don't see that in every center city."

Wes Potter, manager of the Shops at National Place along F Street between 13th and 14th, said investments in downtown represent "a belief in the city, its government and downtown as a viable retail area."

Several large law firms spearheaded the move back downtown and thus gave the neighborhood a sense of respectability as a business address. The Covington & Burling firm opted for offices at 1201 Pennsylvania Ave., while Hines started construction of his $100 million project only after the large firm of Hogan & Hartson agreed to lease 230,000 square feet of the 530,000 available for offices in the building. The zoning law firm of Linowes & Blocher moved in late 1983 from a choice location at Connecticut and L streets NW to Carr's Metropolitan Square complex, also the home for the Miller & Chevalier firm and others.

In addition to being a key downtown developer, Carr is cochairman with McClinton of the joint private-public Downtown Partnership. The group, with a privately funded $500,000 annual budget, is seeking to boost the revitalization effort through shopping promotions, construction of public spaces, incentives for varied land uses and other measures. In Carr's view, the new downtown will not be the mirror image of its old self.

The downtown of yesteryear primarily consisted of small retail shops, and many of the older, deteriorating stores remain in the 25-block sector east of 15th Street. But numerous others are boarded, awaiting the wrecker's smashing blows and the advent of more development. In some cases, only building facades are being saved in a bow to the niceties of a bygone era of architecture, while high-technology interiors are constructed.

"What we'll end up with," said Carr from his office overlooking the White House and the Washington Monument, "is approximately the same amount of space, but the quality and vitality will be restored. The grand old theaters have not been reconstructed, but now you will be able to work, stay in hotels, eat, be entertained and shop. There will be a much greater diversity."

The boom town that downtown has become has not come cheap to developers, nor is it for consumers. One developer, Ted Georgelas, president of John G. Georgelas & Sons Inc., said he has "been a believer" in downtown's reemergence "ever since we started acquiring property five years ago. It didn't take a genius to see it coming."

More tangibly, he also noticed that as he assembled six parcels for construction of an office building that will front on both 14th and F streets, the price of land kept going up, more than doubling from $400 a square foot in 1981 to $900 in 1983.

The new downtown is also one of chic -- and costly -- merchandise. The Sharper Image at the Shops at National Place, catering to the whims and a few necessities of its customers, offers a $695 miniature racing bike that goes 30 miles an hour or a $30 Water Warrior that has turned the look of an M16 machine gun into an adult water pistol.

Down the corridor at the Shops, Michael Darcy, the 25-year-old owner of the Commodore's Place, provides shoppers with the finer leftovers from ships of bygone days, such as $5,000 sets of elegant blue and gold Cobalt china, service for 12, from the Ile de France.

Darcy said he does not sell items in his small shop every day. But he is not disappointed.

"Downtown is now very legitimate," Darcy said. "There's no question it will work. It's got all the ingredients. There's a very good clientele here."

Rick Jones, owner of the three-store Powers & Goode men's and women's clothing chain, said he recently opened an outlet at the Shops because "this is going to be the future shopping area of Washington. With all the attorneys and professional people moving here. . . Washington is becoming more sophisticated, rather cosmopolitan."

Another retailer with an eye on the wallets of the professionals moving to the city's downtown is Hecht's Harris.

"There are 145,000 professional men and women within a 10-minute walk of here," Harris said. "By the end of 1986, there will be 25,000 more."

Kwasi Holman, chairman of the Redevelopment Land Agency, the city's urban renewal governing body, called the opening of the new Hecht's store "a major turning point" in the rehabilitation of downtown. "It indicates that major retailers show the confidence that we have had," he said. "Many cities would be proud of this as we are. It strengthens our marketing effort to attract other large and small retailers. It seems to tie our retail core together."

With the relocation of Hecht's from its previous location on Seventh Street NW, the three major downtown department stores -- Hecht's, Woodward & Lothrop and Garfinckel's -- are now within a four-block area, which retailers and city officials alike believe will prove crucial in luring shoppers back to the center city.

John Fondersmith, the city's chief downtown planner, said that when Hecht's was located on Seventh Street, there was a 3,000-foot walk between there and Garfinckel's on 14th Street, a distance too great for most shoppers to walk and one of the longest retail cores in the country. Now, with Hecht's sandwiched between Garfinckel's and Woodies, the distance has been cut in half. Meanwhile, the city and developer Nathan Landow are trying to entice Bloomingdale's or another major retailer to open a store across G Street from the new Hecht's.

Ellen M. McCarthy, executive director of the Downtown Partnership, said that with "the Shops, Woodies, Hecht's and Garfinckel's, in many ways you'll now have a more convenient shopping trip than at a suburban mall."

As Fondersmith said, "There's going to be a tremendous image change. While we're not trying to replicate a suburban mall, the centralization of major downtown stores is going to make a difference.

"There's the subconscious," he said of shoppers, who might be thinking, " 'If I don't get it here, I'll get it in the next block.' " CAPTION: Picture, The downtown resurgence was signaled in Hect's opening yesterday of its $40 million flagship store. The Washingto Post