America's First Tourists gave Parisians a glimpse of the Reagan style tonight, hosting a dinner that combined some of France's "right people" with a lot of the Left.

It was President and Mrs. Reagan's social curtain-raiser on their 10-day European grand tour, a dinner for 125 at the American ambassador's elegant residence for the "neighbors" from up the street, French President Francois Mitterrand and his wife, Danielle.

Nancy Reagan stole the show from some of the world's most fashionable women by wearing black satin knickers with rhinestone-button cuffs and a short black chiffon overskirt designed by James Galanos.

Mrs. Reagan has been known to disapprove of pants at official functions, so her outfit came as a surprise. But several of the French women also wore evening versions of pants and culottes. Mrs. Mitterrand was more conservatively dressed in a classic white suit with an embroidered jacket and white ruffled blouse.

Mrs. Reagan also wore square-cut ruby earrings outlined in diamonds and a black-beaded multi-strand choker with stones that matched the earrings. When asked whether the jewels were Mrs. Reagan's, Sheila Tate, the first lady's press secretary, said, "Whatever she has on is hers." Asked whether the jewelry was real, Tate shrugged.

Asked what she thought of Mrs. Reagan's knickers, Claude Pompidou, widow of the former French president, said, "She's always very elegant." Earlier, a White House aide joked about the ensemble, "I don't think I'm going to comment."

Reagan, in his after-dinner toast, recalled the long alliance between the two nations and the price Americans have paid. "I think there is no more fitting way to underscore this relationship than to recall that there are more than 60,000 young American soldiers, sailors and marines who rest beneath the soil of France." Noting the times French and American soldiers have fought side by side, Reagan told the story of a young American named Martin Treptow, of whom he had also spoken and quoted in his Inaugural Address, "who left his job in a small-town barbershop in 1917" to fight in France and was killed on the western front. Quoting from Treptow's diary, the president said, "He had written that we must win this war. He wrote, 'Therefore I will work. I will save. I will sacrifice. I will endure. I will fight cheerfully and do my utmost as if the issue of the whole struggle depended on me alone.' "

In his response, Mitterrand recalled that French soldiers fought beside Americans during the Revolutionary War. He talked about the need for peace and said, "It is essential that we should not be, in fact, fighting among ourselves." Mitterrand warmly welcomed Reagan to France, their relationship having progressed to a first-name basis in just a year. The French president at one point addressed Reagan as "my dear Ron." He also told of long talks with Mrs. Reagan last summer in London during the wedding of Prince Charles and Lady Diana, and "indeed we also talked about you," he said, nodding to the president.

Earlier Sheila Tate told reporters that Mitterrand had "personally" invited Mrs. Reagan to Paris when the two met at the British prime minister's lunch after the wedding last summer. "I've had a lot of people ask me why Mrs. Reagan is here," said Tate. The Washington Post reported earlier this week that a highly placed French official said wives are not invited to summit meetings and that Mrs. Reagan had been invited when the French learned she was coming anyway.

Earlier, as the chic crowd arrived in intermittent rain, U.S. Marines in full dress uniforms hurried the women along under red and blue umbrellas. It was every man for himself, however.

The menu was French, the wines American and the ambiance a little of each, set as it was against harpsichord music during the predinner reception and jazz piano during coffee later. The Reagans' guests included most of Mitterrand's top ministers, some of his political critics, a few Americans living in Paris and a dozen or so French aristocrats with fancy titles and clothes.

Claude Pompidou and actress Olivia de Havilland wore short white silk gowns. Simone Veil, former president of the European Parliament, wore a flouncy pink and white dress. The Countess Jacqueline de Ribes wore a midknee-length sleeveless black and white silk.

Other guests included Bernard Hanon, head of French Renault, which owns 46.7 percent of American Motors, former French ambassador to the United States Francois de Laboulaye and Julien Green, 81, a writer and member of the French Academy who attended the University of Virginia.

The White House apparently thought the event so important that it sent Social Secretary Muffie Brandon here a week early to oversee arrangements. According to the White House, these included pink chintz tablecloths on round tables set with State Department china and glassware. The menu was sea bass flambe in fennel, saddle of lamb a la Richelieu, veal, bouquetiere of fresh vegetables, Brillat-Savarin cheese and Bavarian bombe with raspberry sauce. There were three wines: Grgich Hills/Chardonnay 1979; Martha's Vineyard Heitz/Cabernet Sauvignon 1974; Domaine Chandon/Brut Special Reserve.

When the flaming sea bass was brought into the elegant dining room, where two huge Beauvais tapestries hang, the smoke alarms went off. But detective work by alarmed White House aides revealed that it was veal browning in the kitchen that set them off.

In the toasts at the end of the dinner, Reagan tried out his French. "Vive la France. Vive l'America," he said and then corrected himself, saying, "Vive l'Amerique."

It was President Reagan's second big meal of the day, less than 24 hours after he and Mrs. Reagan arrived in the rain. At lunch, President Mitterrand welcomed him to veal with truffles, washed down with three French wines, at the Elysee Palace.

Tonight, on the eve of the economic summit conference at Versailles, the Reagans chose dinner companions from a broad spectrum of French life. Some political observers in the French media see Versailles as "realism," the NATO conference in Bonn as "symbolism" and tonight's dinner as some necessary fluff. It gives the Reagans one last chance to be together before the president heads off to Louis XIV's old spread west of here. The president will be sleeping in the same bed as emperors and kings have before him in the pink marble Grand Trianon, and Mrs. Reagan will not join him until Sunday night when Mitterrand entertains summit leaders at a glittering gala dinner in the palace's historic Hall of Mirrors.

The French have not disclosed the cost of their summit house party, although published reports range as high as $13 million.

Among tonight's guests was Jacques Attali, special adviser to President Mitterrand, who organized this summit. Also there were the French ministers of foreign affairs, agriculture, interior, economics and foreign trade, and the president of the powerful French Employers' Council.

From the world of titles and fashion were several of Nancy Reagan's old friends, including a countess, two marquises and two princesses.

Princess Ghislaine de Polignac has a public relations firm whose clients include the couturier house of Cardin. Another guest, though untitled, was Dreda Mele, also a public relations representative for several Paris fashion houses.

Mrs. Reagan's activities earlier in the day were not announced. She and the president were house guests of U.S. Ambassador Evan Galbraith and his wife, Bootsie. Friday, when the president moves to Versailles, Mrs. Reagan will be more visible; she is scheduled to have lunch with Mrs. Mitterrand, visit a center for blind youths, and be the star guest at an evening reception given by the Galbraiths at the Petit Palais for Americans in Paris.