As France's embattled left-wing president, Francois Mitterrand, strode on stage, thousands of ecstatic Socialist supporters burst out into rhythmic applause. A huge white screen behind the presidential podium was suddenly transformed into a gigantic French tricolor by red, white, and blue floodlights.

The carefully stage-managed scene was evocative of the mass rallies held by Mitterrand up and down France during his victorious campaign for the presidency in May 1981. And that is what it was: the first political rally in what promises to be a bitterly fought campaign for next year's legislative elections.

This country's most unpopular president in over a quarter of a century came to the capital of ancient Brittany in western France tonight to defend his government's unpopular economic policies, renew his links with the Socialist faithful and set out his party's program for the next elections. Alternately emotional and combative, he demonstrated that he could still be a formidable opponent for rightist political parties who already sense victory.

Scarcely consulting his notes during his 75-minute speech, he also proved that he remains one of the country's most powerful political orators.

"We are the France that wins" read a red banner at the end of the packed hall -- a phrase that is likely to emerge as the Socialist party's next election slogan.

Although elections for the French National Assembly are still more than a year away, they are already concentrating the minds of politicians on either side of the ideological divide. The fact that presidential elections are not due to be held until 1988 has raised the specter of a potentially destabilizing constitutional deadlock between a right-wing assembly and a left-wing head of state.

Under the constitution of the Fifth Republic, inaugurated by Gen. Charles de Gaulle in 1958, the president's choice of prime minister must be ratified by the assembly.

The presidential palace laid on a special plane to fly dozens of journalists to Rennes for Mitterrand's first such mass meeting since 1981. The city -- a Socialist pocket in a generally right-wing region of France -- was plastered with posters of Mitterrand.

The emphasis on the phrase "the France that wins" appeared designed to counteract the image of economic and political reverses with which Mitterrand's administration has been tarred since it abandoned its ambitious plans for economic growth in 1982. It is in marked contrast with his more detached 1981 election-winning slogan: "the tranquil force."

Mitterrand claimed that austerity measures applied two years ago to reduce a 12 percent inflation rate and a widening foreign trade deficit are beginning to pay off. The trade deficit, he said, has been cut by half and inflation fell last year to 6.7 percent.

He described the record level of 2.4 million unemployed as "the main accusation leveled against the present government by our enemies" but went on to blame France's recent economic difficulties on the "the oil shock" and "the dollar shock."

His voice rising above the applause, he said: "We will never be pushed away from the path that we have undertaken because we are convinced that it corresponds to the interests of France."

Mitterrand's advisers hope that the improvement in the key economic indicators will enable the Socialists to achieve a respectable result in the next elections. They are also calculating that the scale of the expected swing to the right will be reduced by the introduction of proportional representation in place of the present system of winner-takes-all.

The government's announced intention of changing the electoral system has been denounced by right-wing opposition leaders who are fearful that it might rob them of outright victory in next year's polls.

"You can't change the rules in the middle of the game -- particularly when you are about to lose," complained Jacques Chirac, the head of the neo-Gaullist party, in a television interview last week.

The president used the rally to renew a 1981 pledge to identify France with the aspirations of economically deprived Third World countries. He also called for a greater European effort in space exploration in competition, if neccessary, with the United States.

Mitterrand's program in Rennes was carefully constructed to highlight the importance placed by his government on modernizing France's outdated industrial infrastructure. It included the now obligatory visit to a high technology plant, in this case a computer firm.

In a speech welcoming the president, the director of the Sofrel plant echoed Mitterrand's theme of "a winning France" -- but went on to complain about an increase in corporate profits tax under the Socialist government.

Prior to tonight's rally, the president was given a mixed reception by the people of Rennes -- with cheers mingling with whistles and jeers from a crowd of several thousand as he arrived at the mayor's office. Some demonstrators held banners calling for Mitterrand's "resignation" and young supporters of former president Valery Giscard d'Estaing distributed leaflets predicting "a summer of hope" after "a winter of socialism."

The effective launching of the campaign for next year's elections has coincided with a maneuvering for position among the leaders of the right-wing opposition. The last few weeks have seen the strengthening of a somewhat unlikely alliance between Giscard and his one-time prime minister, Chirac.

Bitter foes when they stood against each for the presidency in 1981, the two are now vying with each other in calling for "the unity of the opposition." The rhetoric is widely perceived as being directed against another former prime minister, Raymond Barre, who is depicting himself as independent of the established political parties.

Barre, formerly one of France's most unpopular prime ministers because of his calls for economic belt-tightening, has established a significant lead over both his principal right-wing rivals in opinion polls.