Formally unveiling the most radical French economic measures in two generations, Socialist Prime Minister Pierre Mauroy presented a program today for nationalizing key industrial and banking groups and decentralizing political power.

Buoyed by a message from President Francois Mitterrand and an absolute Socialist majority in the 491-member National Assembly, Mauroy said, by approving purchase by the government-owned ELF-Aquitaine oil giant of Texasgulf, the American minerals corporation.

Designated for "immediate" nationalization -- according to government officials that means at a special parliamentary session planned for September -- were Dassault-Breguet, the largely military aircraft manufacturer, and the armaments end of the Matra conglomerate.

Stressing the government's case-by-case approach, Mauroy said studies would begin "without delay to take into account the specific situation of those firms."

Also slated for similarly "immediate" nationalization were the Sacilor and Usinor steel companies, which in 1980 alone lost $600 million and have been bailed out by government loans since 1978. For all intents and purposes they are already under state control.

Scheduled, too, for "immediate" nationalization this fall, Mauroy said, were the parent companies of five giant firms -- Compagnie Generale d'Electricite (electrical gear and generators), Pechiney-Ugine-Kuhlman (chemicals and metals), Saint Gobain-Pont-a-Mousson (glass, steel pipes and paper), Rhone-Poulenc (chemicals and manmade fibers) and Thomson-Brandt (electronics and electrical equipment). Minority foreign shareholders were encouraged to remain or sell out to the government.

The three large companies with foreign holdings to be nationalized later are CII-Honeywell-Bull. a computer firm with a 47 percent U.S. stake; ITT-France, a subsidiary of the U.S. multinational telephone equipment firm, ITT, and Roussel-Uclaf, the pharmaceutical firm 59 percent owned by Hoechst of West Germany.

Mauroy provided no details about indemnifying foreign and French shareholders in nationalized firms, but promised a "juridically sound and financially equitable" solution.

Apparently to defuse anxiety, Mauroy said a special interministerial committee would oversee contacts between the government and the firms to be nationalized during the transition before final legislation is voted.

He made no reference to earlier suggestions that in some cases the government would be satisfied with a controlling rather than total interest. The absence of such details, which government officials suggested would be provided in a few days, encouraged opposition criticism that the Socialists were embarked on pie-in-the-sky schemes that were bound to fail.

Running throughout Mauroy's presentation was a conviction that nationalizations would lead to shop-floor democracy, social experiment, technological creativity and new jobs.

Mauroy said the top priority was fighting unemployment, which has idled 1.8 million French workers -- 7.5 percent of the work force -- with further deterioration predicted in the months to come.

After a two-year economic program -- to be followed by a five year program -- the government hoped to create 210,000 new jobs, Mauroy said.