Papal assailant Mehmet Ali Agca today altered large parts of his pretrial testimony in a courtroom performance that provoked the public prosecutor to later accuse him of deliberately destroying his own credibility.

Cross-examined relentlessly by Judge Severino Santiapichi, Agca outlined an entirely new version of an escape plan from St. Peter's Square after the shooting. Other portions of his testimony affected by today's changes included his acquisition of a false Turkish passport, his journey to Rome prior to the assassination attempt, the movements of his alleged Turkish accomplices, the housing for his fellow Turks in Rome, the alleged $1.2 million reward for shooting the pope and the use of hired cars by the papal conspirators.

In previous accounts of the escape plan, Agca insisted that Bulgarian secret agents in Rome had promised to smuggle him out of the country in a diplomatically sealed truck and provide him refuge in Bulgaria. He identified the driver of his getaway car from St. Peter's Square as Sergei I. Antonov, a Bulgarian airlines clerk who has been detained in Italy for the past 2 1/2 years.

Today, Agca said that the Bulgarian escape plan had only been "an alternative" to a plan drawn up by the Turkish right-wing terrorist group known as the Gray Wolves. The main plan, he said, involved escaping in a car over the Alps and hiding out in a "safe house" rented by the Gray Wolves in Austria.

"How many truths are there inside you, Agca?" interrupted the judge when the papal assailant seemed hopelessly caught in his own contradictions. "We are serious people. We are not children. You can't come out with a new truth every time you open your mouth."

In an apparent reference to the two-year pretrial investigation, prosecutor Antonio Marini shouted, "You have led the justice authorities around by the nose for two years."

After Judge Santiapichi told Agca to be "serious," the prosecution's key witness replied: "I thought for a moment that one had to clear the Bulgarians."

"Why did you think that?" interrupted Santiapichi, who had earlier cited passages from Agca's pretrial testimony in which the Turk said he had attempted to "please" Italian magistrates by occasionally inventing evidence.

His anger rising, Santiapichi said, "I do not want you to say things to please me. We are judges. We belong to no one except for this state. Get this into your head and try to tell us the truth."

Caught by the court in repeated contradictions, the Turkish gunman announced at the end of today's session that he would refuse to answer further questions. His testimony has formed the key element in the Italian state's case that Bulgarian secret agents ordered the attempted assassination of Pope John Paul II in May 1981 because of the Kremlin's concern about social and political upheavals in Poland.

"I confirm everything that I have said up to today," Agca told the court. "I cannot reply any more because there is nothing left to say. I cannot invent new things. I have said everything."

After the session, prosecutor Marini told journalists that the month-old trial would continue with the calling of other witnesses and defendants even if Agca refuses to testify. But the prosecutor acknowledged that the credibility of the state's star witness has suffered an important setback.

"If he wanted to destroy his own credibility, he has succeeded magnificently," Marini said, wondering out loud "what strategy" Agca could be following. The unstated implication of his remarks appeared to be that the Turk was trying to create a smoke screen to cover up for his accomplices.

Defense lawyers for the accused Bulgarian insisted that the "Bulgarian connection" to the papal plot was simply falling apart under the judge's incisive questioning of Agca.

The pope's would-be assassin has changed elements of his story almost continuously during the past four years. The trial was adjourned until Thursday.