Lawyers defending Sergei Ivanov Antonov against charges that he was involved in the May 1981 papal shooting have quoted witnesses as saying that the Bulgarian was at his airline office on the day of the attack rather than at the scene of the shooting as the would-be assassin has claimed.

According to detailed but unsourced reports in all leading Italian newspapers this week, the witnesses gave testimony aimed at establishing alibis for Antonov on the day of the shooting and the two days preceding it.

Italian Judge Ilario Martella, who heard depositions Thursday from seven or eight Bulgarian and Italian defense witnesses, must decide within a week whether the alibis are strong enough that Antonov should be released.

The press reports on the hearing have led to speculation in the media and among some government officials that the charges against the 34-year-old Antonov may be dropped and that he may be expelled from the country. Such a move would seriously weaken the theory that Soviet Bloc agents had masterminded the attack on Pope John Paul II because of irritation with his support for the independent union Solidarity in his native Poland.

The strength of the defense's case appeared to depend in part on the reliability of the witnesses. The Bulgarian government has strongly rejected suggestions that its citizens were involved in the shooting, and its employes at the airline office or the Bulgarian Embassy might not be considered to be objective.

Antonov was arrested Nov. 25 after convicted papal attacker Mehmet Ali Agca reportedly said that Antonov and two other Bulgarians went with him to St. Peter's Square on the day of the shooting, May 13, and had helped him on May 11 and 12 to prepare the attack.

The other two Bulgarians implicated in the case, embassy cashier Todor Aivazov and former secretary to the Bulgarian military attache Zhelyo Vasilev, had left Italy before the accusations were leveled and thus have not been detained.

Italian lawyers Giuseppe Consolo and Adolfo La Russa yesterday submitted an eight-page legal brief asking for Antonov's release because of lack of proof of the charge of "active complicity" in the attack. Consolo today expressed optimism, saying that the lawyers were awaiting Martella's decision with "trusting expectation."

Consolo declined to provide details on his witnesses' testimony, but all of Italy's major newspapers provided similar accounts of what was said.

The witnesses were reported to have testified that on May 11, Antonov was busy most of the day at Rome's Leonardo da Vinci Airport handling a shipment of racing bicycles to Bulgaria. On May 12, he was described either as working at his Balkan Air office or carrying out special vigilance duties at the Bulgarian Embassy.

Other witnesses are said to have confirmed that Antonov was in the Balkan Airline office on the day of the shooting, that he learned of the shooting by phone from the mother of one of his employes and that he subsequently rushed to his car to get a radio that he brought into the office and followed reports of the shooting.

Today's press reports also said that the defense lawyers' brief included photographs showing that in May 1981 Antonov did not have the mustache that he now sports. Agca is reported to have described Antonov as having a beard and a mustache, although at one point he is said to have told Martella that the beard could have been false.

The leaks regarding the defense testimony apparently were all from the same unidentified source. They have given rise to speculation that Agca may have deliberately tried to implicate the Bulgarian and that someone may have given him the information necessary to do so, presumably to discredit the Soviet Bloc country. The respected Milan daily Corriere Della Sera said today that this was the defense lawyers' position.

Judicial sources have confirmed that Agca insisted, in his first confession after being captured, that he had acted alone. The question--as yet unanswered--is why he has changed his story and whether it should be believed.

According to press reports, some of which have been confirmed by judicial or police sources, Agca backed up his story of Bulgarian complicity by providing phone numbers and addresses for Antonov and Aivazov as well as descriptions of their Rome apartments.

A spokesman for the Bulgarian Embassy said today that Agca's story, as reported, included several notable inconsistencies. He also suggested that Italian authorities may have ignored diplomatic rules to obtain information about the three Bulgarians implicated in the case.

The spokesman, First Secretary Vassilli Dimitrov, said that reports that the Turkish terrorist had Aivazov's telephone number were meaningless because Aivazov never had a phone. He said that Agca's claim to know the Bulgarians only by code names meant that his knowledge of the embassy switchboard's number would have been useless.

Dimitrov said that Aivazov's apartment is in a building owned by the Bulgarian Embassy that enjoys extraterritoriality, and a formal request by Italian judicial authorities would have been necessary to see the apartment and verify Agca's alleged description. Such a request never has been made, Dimitrov said, adding that the Bulgarian Embassy has sent four diplomatic notes to the Italian Foreign Ministry since September complaining about alleged break-ins in the building.

Investigating magistrate Martella yesterday flew to Munich to question another potential witness in the case, a Turkish businessman who was a former associate of Turk Bekir Celenk. Agca reportedly has said that Celenk offered him 3 million West German marks (about $1.25 million) to shoot the pope.