Turkish gunman Mehmet Ali Agca refused to respond to the judge's questions today after being called as the star prosecution witness in a trial of three Bulgarians and five Turks accused of plotting to kill Pope John Paul II.

Agca, 27, who has already been sentenced to life imprisonment for shooting the pontiff, repeated his claims to be "Jesus Christ" and described a vision he said he had of the destruction of the world.

He said that the attempt on the pope's life was connected with "the third secret" of Fatima -- a reference to a secret message said to have been conveyed by the Virgin Mary to a group of Portuguese children early in this century.

The pope's would-be assassin, who is a Moslem, told Judge Severino Santiapichi that he wanted to wait for the Vatican to comment on his claims before answering questions from the court. When the judge suggested a 15-minute coffee break to allow him to reconsider his refusal, he said he was going to "talk to God."

Agca's renewed outburst was immediately seized upon by defense lawyers as evidence of his lack of credibility as a witness. In pretrial testimony, Agca said he was hired to assassinate the Polish-born pope by the Bulgarian secret services for $1.2 million.

"These kinds of remarks do not surprise me in the least," said Giuseppe Consolo, an Italian lawyer representing Sergei I. Antonov, the only one of the Bulgarian defendants in custody in Italy. "Agca has always been like this. What surprises me is that Italian justice authorities have taken this man seriously for the past three years."

State Prosecutor Antonio Marini insisted, however, that Agca's claims to be "Jesus Christ" did not affect his overall credibility as a witness. He cited testimony provided by Agca Monday on how he came into possession of the Browning semiautomatic pistol with which he shot the pope.

"Up until now, when Agca has talked about facts like how he got the gun, he has been reliable. When he talks about things that are happening in heaven, it is impossible for us to know or not whether he is reliable," Marini told journalists.

The case against the Bulgarians has centered on Agca's ability to provide judges with accurate details of their facial characteristics, apartment layouts and movements. All three Bulgarians have denied ever meeting with the Turk.

Agca's mention of the "third secret of Fatima" referred to a set of messages that the mother of Christ is said to have given to three children in the Portuguese town of Fatima in 1917. The children claimed to have seen visions of hell and persecution of the church in Russia, as well as a third vision that the Vatican has refused to reveal.

The madonna of Fatima is held in particularly high regard by John Paul, who visited the Portuguese shrine a year after the failed assassination attempt to thank God for saving his life.

Today's dramatic exchange between Agca, speaking Italian, and the judge began with the Turkish gunman saying that he would like to draw attention to an important matter that he had not mentioned during a two-year pretrial investigation. According to an unofficial transcript, the following exchange then took place:

Agca: "The attempt on the pope's life is connected with the third secret of the madonna of Fatima. I am Jesus Christ reincarnated. Days are numbered for this generation. No one, neither Americans nor Soviets, will be saved. There will be destruction. Remember that the pope came to see me in my cell in December 1983 . I have spoken with the invisible God who showed me in a vision the crucifixion, the resurrection and the ascension. The pope did not say to me 'How?', he said to me 'When?' " Here Agca raised his voice to a shout. "The pope also said that the meeting with me had been excellent. If I am mad, why should he say that the meeting was marvelous and excellent?"

Judge: "The court is not concerned with absolute truth. It does not seek to interpret absolute truth. I let you speak so as to have all the necessary elements. Now I must ask you not to return to such matters which are not part of the trial. This is the minimum which I can ask you. Now let us return to the pistol. . . . "

Agca: "I cannot reply. I should like to continue tomorrow."

Judge: "I cannot postpone the session. I am not at your disposal."

Agca: "Today I am waiting for a reply from the Vatican. If it remains silent, I will continue to cooperate tomorrow. If the Vatican denies what I have said, I cannot do anything. I cannot continue."

Judge: "You have the right to speak but I cannot agree to a deviation."

Agca: "For today only."

Judge: "I must concern myself with the trial. I have not asked you any questions concerning the Vatican."

Agca: "Today I cannot reply. I do not feel well. I will reply tomorrow."

Judge: "The trial must go on. I cannot put myself at your disposal. Let us stop for coffee. . . . "

Agca: "I am going to talk with God."

After the break, Agca repeated his refusal to answer questions and the court began interrogation of Omer Bagci, a member of the extreme right-wing Turkish group, who is alleged to have smuggled the Browning semiautomatic into Italy. Bagci acknowledged meeting with Agca but said that he was unaware of his real identity.

Agca's court-appointed defense lawyer, Pietro D'Ovidio, said no full psychiatric examination of his client had ever been carried out in Italy. At his first trial in July 1981, Agca threatened to dismiss D'Ovidio as his lawyer if he asked for a psychological examination.

A psychiatric report on Agca carried out in Turkey after his arrest in June 1979 on charges of murdering an influential newspaper editor, Abdi Ipekci, concluded that he was mentally fit to stand trial.

Before Bagci began testifying, the judge accepted a defense request to exclude Agca from the courtroom during the testimony. Defense lawyers argued that Agca might try to make use of details revealed by Bagci in his own testimony later.