The dissident French Communist, waging a campaign against what he regards as the manipulation of his party by Moscow, identified himself by his nom de plume, "Jean Fabien." To prevent the call from being traced, he insisted on speaking from a public telephone booth.

"What is your real name?"

There was a nervous laugh at the other end of the line.

"I cannot reply to questions like that. In a Communist party, you never survive very long if you attack the leadership openly. We have chosen the methods that we consider the most efficient."

The aura of secrecy cultivated by Fabien, who has published a series of internal documents on relations between the French Communist Party and the Soviet Union, has provided France with its latest literary mystery. It is also a good illustration of the dwindling political weight of the reformist faction within the French party.

After enjoying considerable influence in the mid-1970s, when French Communist leaders dabbled with the liberal variant of Marxism known as "Eurocommunism," the reformers seem to have lost hope of weaning the party away from Moscow. Some have resigned in disgust. Others, like Fabien, are using the mass media to wage a rear-guard campaign against Communist leader Georges Marchais, whom they regard as a traitor.

Like many of the reformers, Fabien dates his disillusionment with the party leadership to the 1984 European Parliament elections, when the Communist vote fell to 11 percent. The result of the elections seemed to suggest that the Communist Party, once supported by one French citizen in four, was headed for irreversible political decline.

In Fabien's analysis, the French party is caught in a trap. Having lost the leadership of the left to the more moderate Socialist Party, its leaders do not want to risk a public confrontation with the Kremlin. Their pro-Moscow loyalties, however, are costing them even more votes.

Documents published by Fabien show a steady drift by the French Communist Party away from independent positions, such as its criticism of the Soviet invasion of Czechoslovakia in 1968. The turning point, according to Fabien, came in 1977, when the Soviets dispatched a blistering letter to Marchais accusing the French party of allowing itself to be co-opted into blatantly anti-Soviet positions.

Several months after he received the letter, Marchais formally broke with the Union of the Left, an electoral alliance with the Socialists. After the Socialist election victory in May 1981, the Communists agreed to join a Socialist-led coalition, but they left it in July 1984.

Although Communist Party spokesmen have reacted angrily to Fabien's revelations, describing them as "a filthy operation . . . aimed at destabilizing the party," there has been no formal denial of the authenticity of his documents. Their contents, which coincide with the recollections of other dissident Communists, suggest that Fabien had access to normally secret debates in the party's Central Committee, its principal policy-making body.

Arranging an interview with Fabien, who says he is a spokesman for a group of like-minded Communists, is not easy. It involves elaborate negotiations with his Paris publishers, the submission of written questions, and agreement on measures to safeguard his anonymity that would be worthy of the French Resistance.

A telephone conversation with Fabien, which was arranged to supplement the written interview, was interrupted when he noticed that someone was in line for his pay phone. He phoned back after the stranger had gone.

In the interview, Fabien said that the mid-1970s had been marked by "a real war of independence" within the Western European Communist movement, a largely secret war that eventually was won by Moscow. He described the French Communist Party as the "weak link of Eurocommunism" and the party most susceptible to Soviet manipulation.

"Marchais wanted reform. But he was put in a situation where he had to choose between keeping his post as secretary general and risking a direct confrontation with the Soviet Union. He chose the Soviet Union," Fabien said.

The French defection took the momentum out of the Eurocommunist movement, which was led by the Italian, French and Spanish communist parties. The Spanish Communist Party has since split into pro- and anti-Moscow factions, leaving the Italian Communist Party as the only real ideological challenger to the Kremlin.

Fabien said that, after the waning of the Soviet Union's ideological prestige in Western Europe, the Kremlin used all its "imperial power" to bring western communist parties into line. Soviet tactics included "the use of pro-Soviet forces within each Communist party, threats of division, blackmail and pernicious campaigns against the leaders of rebellious parties."

He added, however, that the United States contributed to the failure of Eurocommunism by its long hostility to the presence of Communist ministers in Western European governments.

The documents published in Fabien's two books, "Secret Conversations" and "War of the Comrades," provide an insight into the internal party debate among French Communists over how to deal with Moscow. They also reveal the ruthlessness of the Soviet leadership in dealing with its "fraternal" parties.

The Kremlin's contempt for the Czech reformers in 1968 is vividly captured in a conversation that Marchais had with the late Leonid Brezhnev shortly before the crushing of the "Prague Spring." In a withering dismissal of the Czech reformer Alexander Dubcek, the Soviet leader described him as "young in age, young in experience, and young in spirit . . . . It's gone to his head."

At another point in the book, a French Communist hard-liner, Jeannette Thorez-Vermeersch, dismisses public opinion with a memorable line: "The masses can be mistaken."

Some of Fabien's conclusions since have been supported by a former Communist leader, Pierre Juquin, who lost his post as party spokesman earlier this year because of his opposition to Marchais. In a book entitled "Self-Criticism," Juquin revealed details about a trip to Moscow by Marchais in 1980 at the height of the "Euromissile" crisis.

Juquin's transcript of the subsequent conversation in the Kremlin has Marchais asking Soviet leaders if their "instruction" is to "annul" a decision by the North Atlantic Treaty Organization to deploy U.S. missiles in retaliation for the deployment of Soviet SS20s.

Disputing Juquin's version of the meeting in Moscow, Marchais said last week that the French Communist Party "did not take orders from anyone."