Pretrial proceedings for three District men charged in the 1980 assassination of exiled Iranian diplomat Ali Akbar Tabatabai began yesterday in D.C. Superior Court under the tightest security measures in recent years.

Regular courthouse guards were reinforced by more than a dozen deputy U.S. marshals and heavily armed D.C. police for the politically volatile case. Law enforcement officials have linked the three defendants in the assassination to a local fundamentalist Islamic group that officials say is violence-prone.

Hundreds of persons on routine business waited in long lines outside the courthouse yesterday morning to pass through metal detectors at the main entrance on Indiana Avenue NW. More than two dozen small knives -- mostly ordinary pocketknives -- were seized during the day. Most courthouse visitors appeared to have no connection with the Tabatabai case.

An additional metal detector was placed in a narrow basement corridor leading to Courtroom No. 1, the most secure courtroom, where the trial will be held before Judge Fred B. Ugast. A floor-to-ceiling bulletproof glass partition separates the participants in the case from spectators.. Marshals also have been assigned to protect Ugast.

Defense attorneys objected to the tight security and to a ruling by Ugast that the jury, to be selected next week, be sequestered for the trial, which may take several weeks. Prosecutors asked for the jury to be sequestered and for some security precautions.

Defense attorneys contend the prosecutors are overreacting. "All the security creates an oppressive atmosphere to try a case," said attorney Bruce B. McHale, adding that it may lead the jury to believe the measures are required "because the defendants are dangerous."

Law enforcement sources said there have been no specific threats, but the decision to tighten security was made, at least in part, because of the political nature of the case.

Law enforcement officials say the three defendants have been associated with a group known as the Islamic Guerrillas of America that reportedly espouses violence against persons opposing the regime of Iranian religious leader Ayatollah Ruhollah Khomeini.

Four men were originally indicted in the Tabatabai killing by a grand jury here last July. According to the indictment, Daoud Salahuddin, also known as David Belfield, who was serving as a security guard for Iranian government representatives here, obtained a U.S. Postal Service vehicle, went to Tabatabai's home disguised as a postman and shot the prominent critic of the Khomeini regime. Salahuddin has fled the country. Law enforcement officials suspect he is in Iran.

The three remaining men are on trial, accused of conspiracy in the killing. Ahmed Rauf, also known as Horace A. Butler, allegedly helped Salahuddin obtain the mail truck and later disposed of the murder weapon. The indictment stated that Ali Abdul-Mani, also known as Lee Curtis Manning, rented the vehicle used for the escape. The third defendant, William Caffee Jr., also known as Kalid, allegedly wiped the car clean of fingerprints and then abandoned it.