Roman taxi drivers call the heavily defended courtroom "il bunker." Italian journalists have nicknamed it "a Tower of Babel."

Both descriptions convey something of the atmosphere inside the converted gymnasium on the outskirts of Rome where the trial of three Bulgarians and five Turks accused of plotting to assassinate Pope John Paul II opened last week. The case has been dubbed "the trial of the century" because of the implications for East-West relations if a Soviet Bloc connection to the conspiracy is proved.

Helicopters hover over the marble-faced courthouse, and police sharpshooters take up positions on the roof as a guarantee against a terrorist attack during the trial, which is expected to last a year. In order to enter the hall, which has housed Italy's major terrorist trials, journalists and court officials have to pass through several security cordons, including a slowly revolving door encased in bulletproof glass.

The first days of the trial have been plagued by technical problems caused by the need to conduct the proceedings simultaneously in Italian, Bulgarian and Turkish. The difficulties have been exacerbated by the failure of the Italian authorities to equip the courtroom with an adequate loudspeaker system or to find competent interpreters.

Disjointed phrases in three languages echoed around the cavernous courtroom as journalists, defendants, lawyers and a large Bulgarian delegation strained their ears to keep track of the proceedings. Presiding Judge Severino Santiapichi, 57, lost his temper several times after being forced to adjourn the trial while the problems were being sorted out.

The patience of the gray-haired, craggy-faced judge, who looks as if he could have been chosen for the part by Central Casting, also has been tested by the bizarre behavior of the prosecution's star witness, Mehmet Ali Agca, who attempted to kill the pope in May 1981. Agca's credibility, a key issue in the case, suffered an early reversal as a result of his claims to be "Jesus Christ" and to have seen a vision of the end of the world.

In the absence of Agca, who has refused to answer the judge's questions until he receives a signal from the Vatican, Omer Bagci, a Turk accused of smuggling the pistol used to shoot the pope into Italy, was interrogated. The Sicilian-born judge has been seeking to prove a link between Bagci and another defendant, Musa Serdar Celebi, the leader of a Turkish rightist organization known as the Gray Wolves in West Germany.

Exasperated by Bagci's obvious reluctance to answer his questions frankly, Santiapichi drew laughter by commenting in mock despair, "It's like being back in Sicily" -- apparently a reference to the Sicilian tradition of omerta, or silence.

Bagci confirmed the essentials of Agca's pretrial testimony on how he received the Browning 9-mm pistol with which he shot the pope. But he denied prior knowledge of the plot and attempted to skate over his own links with the Gray Wolves.

Watching the proceedings from behind the metal bars of one of six white cages that line one side of the courtroom was Sergei I. Antonov, 37, a bespectacled Bulgarian airline clerk identified by Agca as the driver of a getaway car on the day of the assassination attempt. Appearing almost catatonic, he seemed uninterested in the course of the trial and scarcely aware of what was going on around him.

The scene in the courtroom was summed up neatly by a columnist for the daily La Stampa as "a Mediterranean-Balkan trial made up of little men with black moustaches, guttural sounds and American journalists." The U.S. press contingent rivals that of the Italians.

The Bulgarians have brought their own interpreters with them from Sofia and listen to a simultaneous translation of the proceedings through headphones, a more efficient system than that provided by the court. The judge has refused to make this facility available to Antonov on the grounds that Bulgarian-appointed interpreters could be politically biased.

One of the peculiarities of Italian trials is that photographers here have virtual free run of the courtroom. Cameramen and television crews enjoy a five-minute picture opportunity before and after every witness takes the stand and at the beginning of every session.

When Agca was called to testify May 28, a phalanx of photographers jostled for pictures from virtually every conceivable position, including from behind the judge's chair, over the heads of the jurors and crouching behind the witness on the floor.

"Only in Italy would we be allowed as much freedom to record everything that is going on in court," marveled Rudi Frey, a photographer for Time magazine.

Agca, indisputably the media star of the proceedings, bathes in all the attention. He poses jauntily for photographs from behind the metal bars of his cage, smiling confidently and occasionally treating the cameramen to a regal wave.

Agca's outbursts predicting the imminent "end of the world" have provoked a heated debate in the Italian press about whether he really is crazy or merely pretending to be crazy for some obscure reason of his own.

People who are skeptical about Agca's claims of a Bulgarian connection to the papal plot -- the innocentisti, as they are dubbed in Italian journalistic shorthand -- tend to favor the first explanation. Those who think the Bulgarians are guilty, or the colpevolisti, go for the latter.

In an impromptu press conference last week, public prosecutor Antonio Marini said that he thought Agca was trying to send some kind of signal to his accomplices outside prison by temporarily refusing to testify. The prosecution believes that Agca still might be hoping to be sprung from jail in Italy, as he was in Turkey in November 1979 after being convicted of the murder of an influential newspaper editor, Abdi Ipekci.