The French Communist Party broke off its six-year partnership with the Socialist Party today and aligned itself more solidly with the Soviet Union in a move that delivered a sharp blow to Eurocommunism.

The Communists' decision to go it alone on the French political scene was accompanied by a tightening of organizational discipline and an alignment with Moscow's foreign policy positions.

Soviet officials here privately hailed the action as a signal that Eurocommunism-the independent path being charted by Western European Communists-is finished as an effective counterweight to Soviet influence in the world communist movement.

The party's shifts were outlined in a five-hour, 150-page speech by Communist leader Georges Marchais delivered at the opening session of the party congress in the Paris suburb of St. Ouen.

The new line was the culmination of more than a year of party upheaval since many members challenged their leaders' role in the failure of the French left to come to power in the general elections of March 1978.

Dissent inside the French party was fueled by criticism from its Italian and Spanish counterparts. Those parties accused the French leaders of deliberately seeking defeat. The more liberal Italian and Spanish Communists had been counting on a victory of the French left to open the way psychologically for them to share in power in their countries.

Since the left's defeat in France, even the Italian and Spanish Communists have had to start compromising with the Soviets. The Italians and Spanish still have not given any ground, however, on one of the two key elements of Eurocommunism - interanla party democracy, a sphere in which they, unlike the French, have probably passed a point of no return.

The Italian and Spanish parties also preserve their autonomy from the Soviets, the other essential ingredient of Eurocommunism. Nevertheless, the French party, with its actions today, has broken the spirit of a unified French left and has probably undermined the Eurocommunist movement.

Marchais made a slashing attack on Socialist leader Francois Mitterrand and turned down the Socialist's proposal that the two main French leftist parties sould at least sign a "non-aggression pact."

The Communist leader also said that internal party debate has reached the conclusion that life and society in the Soviet bloc are, "overall, positive."

Moscow was represented at the congress by a strong delegation headed by Politburo member Boris Ponomarev. Soviet officials in Paris went out of their way to stress their pleasure with developments.

One Soviet analyst acknowledged that the French party's new tactics are primarily defensive against the encroachments of the Socialists, who had replaced the Communists as the dominant party on the French left.

"It was necessary," said the analyst, "to show that there are clear frontiers between the two parties because all the compromises with the Socialists had led to a blurring of the lines that made it possible for the Socialists to win away Communist sympathizers."

The Communists are still expected to do poorly in the first elections to the European Parliament June 10. Marchasis said that the policy of alliance with the Socialists over the past six years had been largely mistaken. Coming from the man who had led that policy, it sounded very much like classical communist self-criticism.

The Communist Party, he said, will return to its traditional policy toward the Socialists, meaning stiff competition for workers' votes and frequent calls for strikes.

Soviet sources said that Marchais has been invited to Moscow in September. It will be his first trip here in three years. When Soviet leader Leonid Breznev visited Paris in June 1977, Marchais refused to see him.

It was already known that Mitterrand, the Socialist leader, was scheduled to go in September and this was taken as a sign that Soviet relations with Marchais were still cool. Soviet sources are now downplaying the political significance of the Mitterrand visit.

The Soviets nevertheless hinted that they do not yet entirely trust Marchais' conversion. They seem more inclined to trust his chief rival, Roland Leroy, publisher of the party newspaper, L'Humanite.

Just as Italian Communist Party leader Enrico Berlinguer was presured by party members to take the party out of its close association with the ruling Christian Democrats, Marchais had to get in step with his party hard-liners to stay in power.

Marchais and Mitterrand signed a Communist-Socialist joint program in 1972, creating great excitment on the French left. The alliance was generally expected to win national power, even after negotiations to update the joint program broke down in September 1977.

Mutual suspicion and recrimination was the real reasons for that breakdown. Mitterrand told a closed European Socialist conference in Denmark that he intended to reduce the Communists to 15 percent of the vote from their traditional 20 to 25 percent. They would eventually slide into oblivion, Mitterrand said.

In formally breaking the alliance, the Communists apparently made the calculation that their party, with its parallel society made up of trade unions, city governments, party-owned companies and such could continue to thrive in opposition, as it had for 60 years. While currently stronger, the Socialists-in this Communist analysis-were a collection of factions united only by the prospect of power. If defeated, they would return to squabbling with each other-a calculation that proved to be correct.

The Communist leadership's attemtps to blame the defeat of the left on the Socialists met with great skepticism inside the party and with insistent demands for explanations of the leadership's actions-a corollary to demands for real internal party democracry along the lines of the Italian and Spanish parties.

Marchais, who was apparently personally torn, recognized the need for the members to let off steam. He pledged that no party dissidents would be expelled. He also apparently tried to see what he could do about trying to strengthen the party's independence from Moscow.

In what some analysts now think may have been an attempt to find alternate financing for the party, Marchais visited Mexico, Algeria and Romania in rapid succession.

The trips apparently failed.

Marchais' attempts to preserve a semblance of the liberal line probably suffered the severest blow with the unexpected death from cancer of his most important backer in the French party's political bureau, theoretician Jean Kanapa. An old Stalinist, Kanapa served as the moral and political guarantee for Marchais' anti-Soviet line.

Without Kanapa to inspire, guide and protect him, Marchais could no longer resist the pressure from the hard-liners. Marchais was reportedly deserted on several key internal party issues by his own chief protege, Charles Fiterman, who tipped the balance toward Leroy, the hard-line leader whom Moscow seems to like.

At 45, Fiterman is thought to be a comer, and consideration has apparently been given to designating him as Marchais' heir-apparent.

The party hierarchy stemmed the dissident tide. The questioning and debate inside the party cells was gradually confined to party intellectuals in Paris and other university towns. The unhappy Communist blue-and white-collar workers simply stopped attending cell meetings.

Less than half the party membership seems to have taken part in the round of lcoal meetings up to the current congress. The party claims 700,000 members, and recently published statistics pointedly saying that only 13 percent of them are intellectuals.

As delegates were elected to th intermediate levels of the party in the preparations for the congress, the dissidents were filtered out. There should be no trouble adopting the new Marchais line with virtual unanimity. CAPTION: Picture, GEORGES MARCHAIS . . . pressure from below