Ali Akbar Tabatabai, an exiled Iranian diplomat who became one of the most outspoken and highly visible opponents of Ayatollah Ruhollah Khomeini, was assassinated on the doorstep of his Bethesda home yesterday by a gunman posing as a mailman.

The impostor knocked on the door of Tabatabai's home and told another resident he had two special-delivery letters for the 49-year-old former embassy press attache. When Tabatabai came to the door, the assassin pumped several shots into his abdomen from a gun hidden behind a handful of letters, Montgomery County police said.

Soon after the 11:50 a.m. shooting, an ambulance rushed Tabatabai to Suburban Hospital, where he was pronounced dead about 45 minutes later.

Edward Hegarty, special agent in charge of the FBI's Baltimore field office, said last night that "this was obviously a very highly organized and carried-out assassination."

Another high-ranking FBI official said he considered the shooting "a possible act of terrorism" because of "the professionalism, the seeming professionalism of the assassination."

About 30 minutes before the shooting, three men including one dressed as a mailman, abducted a Washington postal worker at gunpoint and commandeered his jeep, which was later found abandoned two blocks from the Tabatabai home at 9313 Friars Rd.

Police and FBI agents were seeking these men last night, but by early today no arrests had been made and police said they had no clear indication of the motive for the killing.

Police spokesmen said last night they do not believe any of the suspects are Iranian.

Should law enforcement officials find the shooting was politically motivated, it would be the first such assassination in the Washington area since 1976, when Chilean exile Orlando Letelier was killed at Sheridan Circle NW by a bomb placed in his car by agents of the military government in Chile.

Tabatabai, who served in the Iranian Embassy during the reign of Shah Mohammad Reza Pahlavi and who founded the anit-Khomeini Iran Freedom Foundation, had often expressed concern for his safety, according to relatives and newsmen.

In an interview with The Washington Post last month, Tabatabai said that the deposed shah's dreaded secret police agency, SAVAK, had been reconstituted by Khomeini and renamed SAVAMA, and had been given the same mandate to monitor and control dissent at home and abroad.

"SAVAK is alive and kicking," he said.

Several times during the last two months, Tabatabai appeared on national television and radio shows to denounce Khomeini's regime and call for the installation of a constitutional monarchy in Iran. He was scheduled to have a live telephone interview with WTOP radio in Washington at 1:30 p.m. yesterday.

Chuck Rich, the WTOP reporter who spoke with Tabatabai late last week and again Monday morning, said the Iranian exile declined to come to the studio to be interviewed. "He said he couldn't come in person because he was afraid for his life," Rich reported.

Tabatabai's twin brother, Mohammad, said yesterday that the former press attache had received a number of telephoned death threats in recent months, which he had reported to the FBI. In one phone call, which was taped, a voice said in Persian, "We will kill you at last."

In recent weeks there have been several incidents of violence against Iranian exiles in Europe. FBI agents were specifically investigating similarities between yesterday's shooting and an assassination attempt against former Iranian prime minister Shahpour Bakhtiar in Paris last Saturday.

In that attempt, in which a policeman and next-door neighbor of Bakhtiar were killed, the would-be assassins posted as journalists in order to get close to the 65-year-old former prime minister.

The State Department released an official statement yesterday saying that they had little information on the Tabatabai assassination and adding, "Of course, we condemn this and all acts of violence n the strongest possible terms.

"Beyond that, we do not know what the motives were in this case."

Bahram Nahidian, a rug dealer and acknowledge -- though unofficial -- pro-Khomeini leader in this country since President Carter severed diplomatic relations with Iran in April, said yesterday, "I'm very happy that this happened. (Tabatabai) is a man who says why doesn't the U.S. bomb all of Iran. He wants Iran to be destroyed."

Nahidian stressed that he is generally opposed to assassinations and said he had no connection with Tabatabai's death.

The assassination plot was set in motion at about 11:15 a.m. yesterday with the abduction of a D.C. mailman according to police.

Tyrone A. Frazier, 31. told police he turned his Postal Service jeep onto Idaho Avenue NW from Massachusetts Avenue when he was flagged down by a black man wearing the blue shirt and gray trousers of the Postal Service uniform.

When Frazier stopped his jeep, the fake mailman -- the same person being sought as the killer -- pulled a hand gun on Frazier, as a van cut in front of Frazier's jeep.

The driver of the van, a white man also displaying a gun, aided the phony postman in herding Frazier into the van where a third man, another white male, forced Frazier to lie face down on the floor.

Thirty-five minujtes after Frazier's jeep had been commandeered, it was parked in front of Tabatabai's expensive, contemporary home in north Bethesda, about six miles from where Frazier was abducted.

Frazier, an eight-year employe of the Postal Service, told police that the two white men held him at gunpoint in the van until 4 p.m., when they dumped him out on Sligo Creek Parkway in Silver Spring. A few minutes later, Frazier made his way to the Sligo Junior High School, where he called his Postal supervisor.

Montgomery County Police Chief Bernard Crook said the fake postman who abducted Frazier "is the suspect" in the assasination. Crook indicated that the impostor drove Frazier's jeep immediately to the scene of the crime from Northwest Washington.

The jeep was found abandoned shortly after the killing, in front of 9207 Fernwood Rd., two blocks from the scene. Police believe the killer used still another vehicle to make his getaway.

Frazier was taken to Montgomery County police headquarters in Rockville where he was questioned by police and FBI agents. A source there said Frazier was "not a suspect" in the case.

Residents of the quiet Bethesda culde-sac where Tabatabai had lived for more than two years said that a Postal Service jeep, driven by a black man, was seen in the neighborhood in mid-morning.

Some time later, police said, a black man in a postal uniform knocked on the door of the Tabatbai home at 9313 Friars Rd., and told the man who answered the door that he had special-delivery packages for Ali Tabatabai.

When Tabatabai came to the door, the fake postman fired several times at short range. None of the three other Iranians in the house was harmed in the attack.

Moments later, one of the Iranians, Shawn Tabatabai, 30, the victim's cousin, broke out a small basement window, crawled through it and ran next door, where he called police.

Shawn Tabatabai was treated at Suburban Hospital for knee lacerations and released yesterday afternoon. He avoided a crowd of reporters who had assembled there to question him.

Several Iranians contacted yesterday said that Ali Tabatabai had been one of the organizers of an anti-Khomeini march and rally scheduled for Lafayette Square Sunday afternoon.

Since the overthrow of the shah and the installation of the Islamic government in Tehran, Tabatabai had taken a leading role among anti-Khomeini, anticlerical Iranian activists in this country.

He founded the Iran Freedom Foundation in the fall of 1979, shortly before the takeover of the U.S. Embassy in Tehran by Islamic militants, and he fervently criticized Khomeini and the holding of the 53 American hostages.

Appearing on the Public Broadcasting System's "MacNeil-Lehrer Report" June 12, Tabatabai gave a detailed description of what he said was the economic and social disintegration of Iran under Khomeini, adding "and of course, the extent of the crimes against the Iranian people, that is most close to our heart."

Tabatabai said his organization neither promoted nor opposed the return of the shah. "We in the United States act as a forum with some 20 other organizations . . . throughout the United States . . . for discussion of Iranian affairs," he said.

"We do give moral encouragement, moral support, if you will, to those national movements in Europe that are actually involved in toppling Khomeini."

In other interviews, Tabatabai said he wanted to promote the unity of the anti-Khomeini groups, including the Paris-based camps of former prime minister Bakhtiar and the more vehemently anticlerical group headed by former Tehran martial law administrator Gholam Ali Oveissi.

Tabatabai decided to head the Iran Freedom Foundation, he once said, because he was a bachelor and other Iranian exiles feared for their families. Tabatabai, who was divorced, has a grown daughter who did not live with him.

The basement of his home in the wealthy suburban Bethesda neighborhood became the headquarters for the IFF. Last Sept. 20, Tabatabai had a burglar alarm system installed there. He told the employe of Rollins Protective Service Co. who installed the equipment that "a death list existed" containing the names of former diplomats who served the shah.

According to the Rollins employe, who asked to remain anonymous, the home also contained many expensive items, including oil paintings and a silver service.

Tabatabai frequently tested the alarm system, according to neighbors, who heard the alarms blaring through the cul-de-sac. Once, when the system was triggered accidentally, police had to be summoned to turn it off, one neighbor said.

The quiet, upper-middle class suburb of Bethesda, which is the home of many diplomats, has been the focal point of three international terrorist attacks in the past seven years.

Letelier died when his car exploded on Sheridan Circle in the District, but the bomb had been planted on his car while it was parked outside his home in the Springfield section of Bethesda.

And in 1973, Israeli Col. Yosef Alon was killed outside his Bethesda home. The Alon crime was never solved, but Arab terrorists claimed responsibility for it.

Police gave the following descriptions of the three men they are seeking:

The gunman, they said, was described as a clean-shaven black man 25 to 30 years old, 5 feet 8 to 5 feet 10 inches tall and 140 to 160 pounds.

One of Frazier's adboutors was described as a white man in his 30s, 6 feet tall, 210 to 220 pounds with receding sandy blond hair and a broad face. He was wearing a white T-shirt and light trousers.

The second abductor was described as a clean-shaven white man, mid-30s, 5-9 to 5-10, 165 to 170 pounds who, police say "displayed a shoulder weapon" during the abduction.