Stoyan Zhulev, Bulgaria's ambassador to the United States, yesterday denied that his country's secret service was involved in the 1981 attempt to assassinate Pope John Paul II, and he accused the western news media of slander.

Zhulev's rare news conference here interested senior U.S. diplomats not so much for its content but because it marked the first time a Bulgarian official has spoken publicly in this country to try countering widespread allegations that Bulgaria was linked to the assassination attempt.

Zhulev presented the Bulgarian position in a lengthy statement and in a question-and-answer session with reporters and guests at the Kennan Institute for Advanced Russian Studies of the Smithsonian Institution.

U.S. officials speculated that the government in Sofia may feel confident that the Italian investigation into the papal shooting has gone about as far as possible without turning up hard evidence of a Bulgarian link, and that a counterattack is in order.

The Bulgarians, one official said, know that they are still under suspicion by western countries in the shooting and that the Italians are the only ones who can clear them. They may launch a public relations campaign to ease the suspicion, the official said.

Top U.S. officials also reportedly believe that the Italian investigation has all but run its course and that, while there is circumstantial evidence pointing to Bulgarian complicity, it is not likely that any "smoking gun" or hard evidence will be found.

Zhulev sharply criticized Italian authorites for keeping Sergei Antonov, a Bulgarian airline official, in jail for seven months without an indictment and without hard evidence against him. Zhulev claimed that Antonov's health is deteriorating because of his jailing, and said that Antonov has been threatened with more years in prison without formal charges.

"What would be the reaction of public opinion" in the United States, Zhulev asked, "if Bulgaria had been proceeding in such an outrageous manner with an American?"

The thrust of his comments, however, was a sharp attack on the western news media for what he called a slander campaign against Bulgaria.

The main witness in allegations involving the Bulgarians is Turkish terrorist Mehmet Ali Agca, jailed in Italy after reportedly confessing to the shooting and being tried and found guilty. He subsequently implicated several Bulgarian officials in Rome, including Antonov, as participants in an assassination plot.

The Bulgarians claim that Agca's testimony against them was planted and that it contains several inconsistencies.

Ilario Martella, the Italian magistrate handling the case, is widely respected in the West and has twice won authorization from Italian courts to continue detaining Antonov because of the gravity of the case. Two other officials implicated by Agca had already left Rome for Bulgaria.

Agca has testified that he spent several weeks in Sofia, the Bulgarian capital, in 1980, and U.S. officials say his passport includes Bulgarian stamps.

Asked yesterday why Agca was in Sofia, Zhulev said, "We don't have any proof" as to whether he was there at that time. Agca reportedly entered Bulgaria on a forged Indian passport bearing the last name "Singh." Zhulev said investigations have showed that 19 Indians with that name were passing through Bulgaria about then.

He also said political assassinations run counter to Marxist theory, which contends that individuals are not decisive in shaping history. Asked about the assassinations of anti-Stalinist Leon Trotsky in 1940 and of Hafizullah Amin, leader of Afghanistan before the 1979 Soviet invasion, Zhulev said such examples are "irrelevant."

In an apparent reference to a lengthy New York Times report in April that a Bulgarian defector in France, supposedly a former member of the secret service, had provided evidence of complicity in the plot, Zhulev said the sources were criminals who had never been in the secret service.

Privately, high-level U.S. intelligence officials also put no credence in the source mentioned in the account.

Zhulev also attacked a recent Washington Post report detailing activities of the Gray Wolves, an extreme right-wing Turkish terrorist group that has a sizable Turkish worker population.

Zhulev said the article depicted Agca as a "renegade" from this group. Zhulev contended that this suggested Agca acted for communist groups rather than for the Gray Wolves, with whom Agca had at least some relationship.