The French leftist opposition led by the Socialists and Communists won a thin victory yesterday in the first part of a two-stage election to be concluded next Sunday.

Communist leader Georges Marchais, surrounded by the other 20 members of the French Communist Party's political bureau, immediately posed conditions for organizing a united leftist front in the runoffs that have already been rejected by Socialist leader Francois Mitterrand.

The Communist leader proposed a leftist summit meeting Monday to negotiate a unity agreement. The other parties accepted.

The closeness of the vote, however, casts doubt on whether the leftist alliance can win a majority of National Assembly seats next week in view of the recent sharp disagreements among the group's three parties. It is unlikely that all the Socialist voters can be counted on to vote for Communist candidates who came out ahead of Socialists.

With about 75 percent of the vote counted, the left was getting fractionally more than 50 percent of the ballots, with the parties backing President Valery Giscard d'Estaing getting 46 percent and the rest of the vote going to minor parties such as the environmentalists.

Although the leftist victory was significantly thinner than had been predicted by the public opinion polls, it was the best result that the French left had obtained since 1945.

With 23 percent of the vote, the Socialist Party for the first time since World War II did better than the Communist Party, which got 21 percent - its apparently irreducible minimum of the electorate.

The Socialists also became the largest party in France, and with their close allies, the Radicals of the Left, they won more than 25 percent of the vote. Other leftist parties got about 4 percent.

The Gaullist Party maintained its position as the largest party on the right with about 22 percent of the vote, a loss of 1.5 percent compared to the last legislative elections, in 1973.

The united Giscardists and centrists, a hurriedly patched-up party formed just a few weeks ago under the name Union of French Democrats polled a surprisingly strong 20 percent. Other conservative groups gave the government side another 4 percent of the vote.

A number of observers attributed the showing of the Giscardists and the unexpected closeness of the results to Giscard d'Estaing's last minute appeal on national television. He warned that economy could not support the fulfillment of the left's campaign pledges.

Even without the Communists demanding tough negotiations, prospects for a leftist victory in the runoffs seemed reduced because of the closeness of the vote.

Socialist leader Mitterrand had been demanding that the Communists and Socialists withdraw in the runoffs in favor of whichever candidate led in yesterday's voting.

The Communists demanded the updating of the joint program they signed with the Socialists in 1972 and recognition that the Communists would get Cabinet post in proportion to their votes yesterday. They also demanded that they not be excluded from key Cabinet ministries.

Marchais repeated all those demands in firm tones last night. Robert Fabre, the leader of the Radicals of the Left, could be seen gulping and loosening his necktie on television as Marchais finished speaking.

Mitterrand has already pledged that Socialist candidates will step down for Communists who are ahead whether or not the Communists reciprocate.

The government parties have also made a similar pledge to withdraw automatically in each other's favor.

Another problem for the left is that the electoral map is cut up so that rural and middle-class districts have heavier representation in the National Assembly than more populous working-class districts.

Although final results were not in, it looked as if only about 50 of the 491 assembly seats would be filled by candidates who got more than half of the vote. Among those elected outright yesterday were Prime Minister Raymond Barre (running for the first time), Gaullist leader Jacques Chirac, Assembly President Edgar Faure, former prime ministers Michel Debre, Jacques Chaban-Delmas and Pierre Messmer and Justice Minister Alain Peyrefitte.

Following the usual pattern, few leftists were elected on the first round. Mitterrand was among those forced into a runoff.

There was a record turnout of 83.5 percent of the voters, a particularly impressive showing since the electorate of 35.4 million is also the largest ever called upon to vote in France. For the first time, it included millions of 18- to 21-year olds.

The turnout showed that although the voters were apparently turned of by the length and repetitiveness of the campaign, they did not lose interest in the outcome.

The election confirmed that for the first time under the Fifth Republic founded in 1958 by Charles de Gaulle, the French political checker-board is fairly evenly divided among four groups - Gaullists, Centrists, Socialists and Communists.

Previously, the Gaullists had been clearly predominant over a large number of small moderate groups on the right, and the Communists far outdistanced the Socialist Party on the left.

The results also confirmed the basic stability of the almost even split of the French electorate between left and right. The factor that gave the left its edge yesterday was an increase by the Socialists of 4 percent over their 1 percent vote in 1973.

Marchais, the Communists leader, said that the left would have gotten better results if the other parties had accepted his demands in the autumn that they renegotiate their joint government program.

"The left," said Marchais, "must now answer two questions: What programs? What government?" The Communists have been saying all along that they were unwilling to be the junior partners of a Socialist Party that would follow a reformist line like the ruling West German Social Democrats or the British Labor Party government.

If the Communists insist on a mathematical split of cabinet posts based on yesterday's results, they would be entitled to slightly more than 40 percent of the positions.

Prime Minister Barre said of Miterrand, "The first secretary of the Socialist Party is confronted with a Communist party that he claimed to dominate and which is now laying down preconditions."

Even if the Communists agree to withdraw their candidates for Socialists who outpolled them yesterday's results were so close that the Communists could now get away with having a few hundred voters stay home in close races where that would make the difference. There is ample precedent for such tactics by the Communists.

In an obvious reference to the tendecy of the less-disciplined Socialist voters not to follow party recommendations to vote for Communists, Mitterrand said last night, "I address to my Socialist friends a message of determination and of hope. United we will carry the victory."

Prime Minister Barre concluded last night by saying, "Nothing is lost but nothing has yet been won."