Defense attorneys in the trial of three men accused of conspiring to assassinate Iranian exile Ali Akbar Tabatabai attacked the credibility of the chief prosecution witness as they concluded their case yesterday in D.C. Superior Court.

The attacks culminated two weeks of attempts by defense lawyers to poke holes in the testimony of the witness, Al Fletcher Hunter, also known as Abu Bakr Zaid Sharriff. Hunter is a confessed coconspirator in the assassination of Tabatabai, an outspoken opponent of the regime of the Ayatollah Ruhollah Khomeni.

Hunter, who testified under a grant of immunity from prosecution, is serving a 6-to-18 year prison term for a D.C. burglary last year. Defense attorneys have been emphasizing that Hunter's role in the conspiracy was far greater than that of any of those on trial.

Tabatabai was shot to death July 22, 1980, in the doorway of his Bethesda home by a man disguised as a postman. The three men on trial before Judge Fred B. Ugast are charged with various counts of aiding the triggerman in carrying out the assassination and the escape.

Authorities believe the accused assassin, David Belfield, also known as Daoud Salahuddin, has fled the country and is now living in Iran. The three men on trial are Ahmed Rauf, also known as Horace Butler, Ali Abdul-Mani, also known as Lee Curtis Manning, and William Caffee Jr., also known as Kalid.

Yesterday, as part of the defense effort to discredit Hunter, Charles Stowe, Hunter's attorney during the burglary trial last year, testified that Caffee, who had been convicted earlier in that same case, told him he refused to corroborate Hunter's explanation of the burglary because it was incorrect.

"Caffee said, 'That's not what happened,' " Stowe said. As a result, Stowe said he decided not to call Caffee as a witness for Hunter. Hunter has accused Caffee of assisting in the Tabatabai assassination by among other things helping to abandon the getaway car in the District.

Rauf's attorney, Thomas Abbenante, called as a witness an Arabic teacher, Abdul Rabb, who Hunter testified had accompanied him once to Rauf's apartment, described by prosecutors as the "launching pad" for the assassination. Rabb acknowledged that he knew Hunter, but said he never went with him to Rauf's apartment.

Bruce McHale, Abdul-Mani's lawyer, presented 10 character witnesses, all of whom testified that Abdul-Mani had a good reputation. Abdul-Mani, who is accused of renting a car and then loaning it to Salahuddin for his getaway, is also charged with perjury for denying to a grand jury that he loaned the car or that Salahuddin reimbursed him for it.

Judge Ugast said both sides would present closing arguments today, and the case would go to the jury Wednesday.