Francois Mitterrand today became the first Socialist president of France's Fifth Republic and immediately sought to reassure the country in a series of symbolic and political gestures that change would be coupled with continuity.

In a rare display of punctuality, Mitterrand arrived at the presidential palace at 9:30 a.m. for a meeting of nearly an hour with his predecessor, Valery Giscard d'Estaing. During the meeting, Giscard's staff said, he passed on the command and control codes for the French nuclear force.

Then, before a small invited audience in the Elysee Palace's ornate Hall of Festivities, Roger Frey, the president of the Constitutional Council, officially proclaimed Mitterrand to be elected as president.

Immediately after the ceremony, Mitterrand named the generally reassuring Pierre Mauroy, portly 53-year-old mayor of the northern industrial city of Lille, as prime minister to lead the Socialist Party in the coming legislative elections.

In a day full of moving moments, perhaps the most poignant came when, while shaking hands with the Elysee guests, Mitterrand came upon former premier Pierre Mendes-France, 74, the man who for years had incarnated the hopes of the noncommunist left and whose eight months in power in the mid-1950s are widely regarded here as the epitome of effective government.

Mitterrand warmly kissed the pale, aging lefist patriarch on both cheeks. To Mendes-France, who could not contain his tears, Mitterrand said: "If I am here today, it is thanks to you. It is the justification for so many years of effort that you were the one to begin."

In a dramatic incident of another kind, before the ceremony began, Mitterrand accompanied Giscard to the front door and watched him leave the palace on foot. As the defeated leader emerged into the street, he was nearly mobbed by a hostile crowd of Socialist supporters shouting slogans such as "Give back the diamonds" -- a reference to campaign criticism of gifts he had received -- and "Out you go."

Police rushed him around the corner to a waiting car that took him to his residence, where a crowd of a thousand mostly young Giscardist faithful sang, "It is only an au revoir, Giscard," to the tune of "Auld Lang Syne."

At the ceremony, Gen. Andre Biard, hurriedly named grand chancellor of the Legion of Honor, France's most prestigious public order, handed Mitterrand the massive gold necklace signifying the president's position as its grand master. Biard was appointed this week to replace Gen. Alain de Boissieu, who had publicly proclaimed during the election campaign that he would resign rather than decorate Mitterrand.

In a brief speech to the assembled political and religious leaders, Socialist faithful and journalists in the hall where President de Gaulle used to conduct his press conferences, Mitterrand said there had been only one victor on the day of his election -- "It is hope. May it become the best shared thing in France."

"As the president of all the French people," said the man who had devoted the last 23 years to combating the center-right majority that de Gaulle had placed in power, "I want to gather them for the great causes that await us and to create in all circumstances the conditions for a true national community."

For the third time in French history, he said, after the Popular Front government of 1936 and the liberation from Nazi occupation in 1944, "the democratically expressed political majority of the French people has just identified itself with its social-class majority."

What higher calling for France could there be, he asked rhetorically, than "to achieve a new alliance between socialism and freedom." France, he continued, "will say with force that there can be no true international community so long as two-thirds of the planet continues to exchange its men and its goods for hunger and contempt."

Mauroy is scheduled to announce Friday a transitional Cabinet composed of Socialists and other close Mitterrand supporters.The president is then expected to dissolve the National Assembly with its center-right majority and to call elections for June 14 and 21. The Socialists hope to dominate the new parliament, cutting into both the Communist representation and the present majority.

In another move apparently designed to reassure the armed forces and to show that he means to retain the essentials of the de Gaulle heritage of French independence, Mitterrand named Air Force Gen. Jean Saulnier, commander of the nuclear strike force for the last two years, as the president's personal military chief of staff at the Elysee.

While a number of leading Socialists expressed dismay over the treatment Giscard got outside the Elysee, others expressed shock over some of his statements in the past 24 hours.

"The voice of conscience is stronger than the favor of the moment," Giscard said of Mitterrand's election in a solemn statement to his last Cabinet meeting yesterday.

In an interview with the magazine Paris-Match, his first since his defeat, Giscard displayed the depth of his bitterness. He said that since being informed of the results on May 10, he had not once listened to the radio, watched television or read a newspaper.

"I suppose it hurt too much," he said. "I imagined what the commentaries could be." He spoke of his feelings of "uselessness" -- "If I had to sum it up, I'd say, 'It's too stupid.'"

He spoke of the "blind" desire for change.

"I think that millions of French people are in an astonished stupor. They wanted what happened, but the consequences frighten them. I think they vaguely understand that what they did is not for the good. . . . At the moment when people must hold tight, everything gives way. . . . What was there to do since the French people were ready to give up?"

Mitterrand was joined by Gaullist leader Jacques Chirac, the mayor of Paris, in ignoring such thoughts today. Chirac, whom Giscard has in effect accused of betrayal, was prominent among the 500 people who joined the Socialists at the Elysee this morning. He greeted Mitterrand at the city hall in an elaborate ceremony in which the two men exchanged speeches showing that they intend from now on to fight out France's political future between them, ignoring Giscard and his bitterness.

The Gaullist leader seems to have conceded in advance that the Socialists will do well in the parliamentary elections, saying it is important to elect a strong Gaullist-Giscardist group in the Assembly as "a counterweight" to the Socialists.

"France," Chirac said to Mitterrand, "has demonstrated its determination to have changes. It is a determination that should be respected. Much remains to be done. . . . Realism in action is more necessary than ever."

Mitterrand seemed to demonstrate that he intends to approach foreign affairs from a socialist angle by inviting a number of foreign Socialists to take part in today's ceremonies, a departure from tradition. Among the guests were West Germany's former chancellor Willy Brandt, who is head of the Socialist International, former president of Senegal Leopold Senghor, Portugal's former premier Mario Soares, former premier Olof Palme of Sweden, Greek Socialist leader Andreas Papandreous and Greek actress-politican Melina Mercouri.

The new president finished his long day with a ritual visit to Pantheon, the resting place of Franche's national heros. There, in the heart of the Latin Quarter, a stone's throw from the Sorbonne University, he encountered the day's most ardent crowds, full of students and young foreigners.

The National Orchestra of Paris played Beethoven's Ninth Symphony" with its "Ode to Joy" sung by a chorale as Mitterrand walked alone into the marble halls carrying three red roses, the symbol of the French Socialist Party. He laid one each on three tombs, including that of Jean Jaures, one of the founders of French socialism, assassinated on the eve of World War I, and that of Jean Moulin, chief of the resistance movement inside France, tortured to death by the Nazis.

When he emerged, the rain that had been threatening all day, started with great intensity, but the crowd at the Pantheon stayed under its umbrellas to listen respectfully as the chorale sang the Hector Berlioz version of the "Marseillaise" and to cheer Mitterrand wildly at the end. Young people took over the Latin Quarter to dance into the night.