The French capital took the first major step today toward filling the gaping hole left in the center of the city when Les Halles, the vast central food market that Emile Zola called "the belly of Paris," was torn down.

One part of the site where generations of tourists ate onion soup while watching muscular workers hauling sides of beef, Paris Mayor Jacques Chirac inqugurated a chic, ultramodern shopping center ranged around an open-air plaza three stories below street level.

Called the Forum of Les Halles, the center's 250 shops include a large number of prestige boutiques such as Yves Saint Laurent, Pierre Cardin and Ungaro.

This is only the first and least contested step in the transformation of the site of Les Halles. Most of the surface area remains to be developed and the many conflicting plans, which have been under study and debate for 20 years, have been turned into a major political football.

President Valery Giscard d'Estaing wanted a formal park along the lines of the Tuileries Gardens at the Louvre. But Chirac, also the Guallist party leader, almost automatically takes the opposite tack from anything that Giscard wants. He said that as mayor, he considers himself the chief architect of Les Halles, and that he wants something closer to Coney Island, a place "where you can smell the French fries."

Rival plans still under debate get away from some of the more nakedly business-oriented schemes, including a Wall Street-style international trade center the late President Georges Pompidou favored.

Nevertheless, the new $150 million Forum, whose arched glass outer walls recall Beauborg, the nearby Pompidou arts center, did not escape the barbs of architectural and political critics.

"In Biblical times," said architect Nicholas Topaloff, "The money changers were chased from the temple. Now, we build temples for merchants." Use of the word forum was cheating, he said. "A forum was where people gathered to talk and debate, not just to buy," he said.

Yet, he conceded, "there is a valid architectural concept, like it or not. I just think architecture with a big "A" should aim higher and be more concerned with human relations."

Built largely with public funds, the Forum's courtyard and the arched tops of its ribbed outer walls cover five of the 15 acres left open in the central part of Les Halles when the 12 huge hangars of the market were torn down. It had been the site of the central market since the 12th century.

A serious attempt was made to rent the Forum to culturally oriented businesses. A quarter of the commercial floor space went to the FNAC, a discount book, record and home appliances company controlled by cooperative associations with a total of three million members. It specializes in presenting broadly attended cultural debates at its stores.

The Forum also includes six movie theaters -- two of them art houses -- an activities center for children, and a new annex of the Grevin, Europe's best-known wax museum next to Madame Tussaud's in London. The Grevin inaugurated its Forum branch with a series of Belle Epoque tableaus including figures such as scientist Louis Pasteur and writers Victor Hugo and Jules Verne, and turn-of-the-century scenes like the Moulin Rouge, home of the can-can girl, and the construction of the Eiffel Tower.

For all that, the Forum is still what an American would recognize as a shopping center. Some participating merchants expressed fear that once the initial excitement is over, there will not be that much business, even though the terminals of new suburban rail lines and a new subway station are expected to bring a million passengers daily to the Forum's mile of shop windows. The Forum also has going for it the prestigious name of Les Halles, the great drawing power of FNAC, its central location and its proximity to the Pompidou center, which has replaced the Eiffel Tower as the single most visited Paris attraction.

Already the surrounding neighborhood has become one of the most fashionable and high-priced residential areas in the capital.

The Forum's expected commercial success already has the capital's leftist opposition complaining that tax money was devoted to aiding financial interests rather than to building badly needed low-rent housing projects.

Jack Lang, the Socialist Party's shadow minister of culture and a leading Paris city councilman, called the Forum "the victory of money over beauty and the happiness of the city dwellers." Along with the rest of his party's city council members, he boycotted the inaugural ceremonies.

This reaction is only the opening skirmish over what happens to the rest of the Halles space. Saying "the time for reflection is over," Mayor Chirac added that the rest of the work must be finished between now and 1983. In addition to green space, he favors building a hotel and office space. But, he is careful to note, his plans call for the creation of Europe's largest automobile-free pedestrian zone, stretching westward from the Palais Royal, home of the Comedie Francaise, to the 17th-century Place Des Vosges, home of Richelieu and Victor Hugo.

Lang has already served notice that having lost the battle to preserve the original market pavilions, he will now center his fight in the city council on preserving the vistas opened up by their removal. Two Renaissance landmarks, the Saint Eustache Church and the unique Foundation of the Innocents, which were dwarfed and hidden by their surroundings, have emerged as major architectural attractions. Large numbers of Parisians do not want them to be hidden again. The question is whether the mayor and chief architect of Paris, who insists on making his decisions in splendid Gaullist isolation, will give them a chance to make their case.