Defense attorneys in the trial of three men charged with conspiring to assassinate Iranian exile Ali Akbar Tabatabai attacked the credibility of the chief prosecution witness yesterday, drawing from him an admission that he had committed perjury in anunrelated trial last year.

Tabatabai, an outspoken critic of Iranian leader Ayatollah Ruhollah Khomeini, was shot July 22, 1980, at his Bethesda home. The chief government witness, Al Fletcher Hunter, also known as Abu Bakr Zaid Sharriff, has testified that he and the threedefendants each took part in the elaborate execution-style killing. Hunter has been granted immunity from prosecution.

Yesterday, Hunter acknowledged under cross-examination that he had lied on the witness stand when he was on trial last year for burglary. He subsequently was convicted of the burglary charge and is serving a prison term of six to 18 years. Hunter was arrested for the burglary in the spring of 1980 and was free on bond at the time of the Tabatabai slaying. His burglary trial was held in the fall of that year.

"No, I didn't," Hunter said yesterday when defense attorney Thomas Abenante asked if he had told the truth at the burglary trial. To another question, Hunter replied that he had committed perjury at the trial. He did not elaborate, and defense attorneys did not press him for details.

It is one of the trial's ironies that Hunter, who enjoys immunity, has admitted greater involvement with the slaying than is alleged of any of the three defendants. Hunter has described how he helped set up the assassination and later drove the alleged triggerman -- David Belfield, also known as Daoud Salahuddin -- to Montreal. Belfield, a former security guard with the Iranian Interests Section of the Algerian Embassy here, is believed to have fled to Iran.

In the current trial before Superior Court Judge Fred Ugast, Horace Butler, also known as Ahmed Rauf, is charged with helping Belfield obtain a U.S. Postal Service truck as part of a ruse in which Belfield drove to Tabatabai's home dressed as a mailman and lured the Iranian exile to his front door, where he was shot. Butler also is charged with disposing of the murder weapon, a 9mm pistol.

Lee Curtis Manning, also known as Ali Abdul-Mani, is charged with renting the car in which Hunter and Belfield drove to Montreal. William Caffee Jr., also known as Kalid, is charged with wiping fingerprints off the car after Hunter drove it back to Washington, and with subsequently abandoning it.

Abenante, who represents Butler, and attorney Bruce McHale, who represents Manning, sought to impugn Hunter's motives by eliciting his testimony that he offered to cooperate with prosecutors only after receiving the unexpectedly stiff six-to-18-year sentence for burglary. Hunter testified that he had expected to receive just probation for the burglary, which occurred at a District liquor store.

"I wanted to get out of jail," he admitted, "but that wasn't the reason I started talking."

Under cross-examination, Hunter said prosecutors have granted him immunity from any charges not only in the Tabatabai slaying, but also in his acknowledged perjury in the burglary case, as well as arson and firearms charges in Virginia, and six bank robbery charges in Maryland.

Defense attorneys also established minor inconsistencies in Hunter's testimony. But the outlines of his account of the events leading to Tabatabai's death remained consistent. He continued to maintain that the three defendents were involved.

In the most dramatic moment of the unusual Saturday court session, Hunter was asked why he was prepared to go along with the executions of Tabatabai and other unnamed anti-Khomeini Iranian exiles.

"For the Islamic revolution," he replied.

McHale then asked if he had been prepared to carry out the killings himself.

"If it had to be, yes," Hunter replied. "That's how I was feeling."

The trial resumes on Monday.