Two dozen models lined the runway in what appeared to be executioner's suits. With a karate shout from the designer, louder even than the piercing rock music, the models snapped off their helmets. Another cry and they snapped off a sleeve. Soon they were standing in brightly painted sweat shirts and leather pants.

It happened today at the Louvre where the ready-to-wear fall fashion shows of French designers, for the first time, are being held in tents in the museum's courtyard--thanks to the collaboration of the Ministry of Culture and the country's fashion industry. More than 500 buyers from stores around the world and 1,200 members of the press have assembled for the five days of showings.

"It may be one of the few good things French President Franc,ois Mitterrand has done," said designer Philippe Heim, as he watched women in St. Laurent suits and punk-clad youths pass a metal barrier into the Anne-Marie Beretta show. "It is the first time the French government has been directly involved with fashion."

Suddenly, the Louvre was no place to find quiet. Inside, Finance Minister Jacques Delors urged a visitor to "come sit by the window." Just then, from the courtyard below, rock music from the Kansai Yamamoto show began to blast. "Well maybe this is not a good place to sit," he said, leading his guest to another room.

Meanwhile, Karl Lagerfeld presented his biggest and most elaborate collection for Chloe', with lots of black, and black with splashes of bright color, and the elaborately beaded and embroidered black dresses produced by French artisans. France Andre'vie also built her collection on black, with oversized jackets, Chesterfield coats with black velvet collars and even black-on-black clown suits as a finale. She added a bright slash of color with a Magic-Marker stripe across the model's face.

The only designer so far to bow to the impressive surroundings with his designs was Emanuel Ungaro, who updated the traditional artist's smock with snug black leather mini skirts, and put models in black berets to underscore the painter's look. He was less successful when he borrowed costume ideas from varied periods depicted in paintings nearby.

Besides the recurring themes of black, big bat-wing sleeves, blouson shapes and big collars and capes, designers also are uniformly indecisive about hemlines. Most are opting to show all lengths, one reason all the shows are running late.

To celebrate the new-found camaraderie between government and fashion designers, buyers and press were invited to a kickoff party in the museum last night.

"I feel very much at home," said Yves St. Laurent as he looked down the long marble colonnade leading to the Louvre's art gallery. Ten-foot white floral arrangements dominated tables of tea sandwiches and pastries in front of the arched floor-to-ceiling windows of the building that was once the home of French royalty.

St. Laurent was the center of attention as designers Mme. Gre s and Sonia Rykiel came up to kiss him, followed by Paloma Picasso, all recorded by photographers and guests who had formed a circle around them.

Minister of Culture Jack Lang and Danielle Mitterrand, wife of the French president, were led through the crowd by Pierre Berge', St. Laurent's partner and current head of the French fashion association, and Edmonde Charles-Roux, wife of Interior Minister Gaston Defferre. Charles-Roux, the former editor of French Vogue, and Berge' introduced designers in the crowd to the shy Mme. Mitterrand and the gregarious Lang.

Mme. Mitterrand, who was wearing a dark royal blue Louis Fe'raud suit with stand-up collar, white blouse and black high-heel pumps, no makeup and no jewelry except her wedding band, smiled at each new introduction. But the French first lady, who still buys most of her clothes at her favorite boutique, Torrente, or at Louis Fe'raud, across the street from where she works in the Elyse'e Palace answering letters, was not very comfortable in the almost totally unfamiliar crowd.

At the far end of the football-field-size hall, Berge' greeted guests on the landing below the bronze "Winged Victory of Samothrace." Berge' saluted designers as the "alchemists of modern times" and spoke of a proposed center for fashion in Paris. Lang suggested that the fashion business was important to France as an industry, "and shows us not only as a country of justice but of beauty."

For the past several years, the French ready-to-wear shows have been like a traveling circus with tents put down one season in the old market area near the Beaubourg Museum, and another time alongside the Porte de Versailles exhibition center on the edge of the city. Last season buyers and press trekked, much of the time in the rain, to the Jardin d'Acclimatation in the Bois de Boulogne for the showings.

"To associate fashion with something as artistic as this museum is absolutely fabulous," said Mme. Gre s.

One American retailer, getting his first taste of France under the socialists, was quick to express approval. "If this is what socialism is all about," said Jack Schultz, executive vice president of Bloomingdale's, raising his champagne glass, "they are getting something for their money. I love the socialistic champagne."