U.S. embassies and diplomats in the Middle East over the past decade repeatedly have been the targets of mobs and terrorists acting often to protest American support for Israel or for unpopular local regimes.

But the car-bomb explosion that wrecked the U.S. Embassy in Beirut today marks a new twist in the tactics of terrorism against the American presence in the region. It was the first occasion in recent times that a bomb was used to blow up an embassy and to try to kill as many American diplomats as possible.

The attack raised fears of similar terrorist acts as frustration mounts over lack of progress in evacuating Israeli troops from Lebanon or in launching new overall peace talks.

"We can expect to see more of this kind of thing in the future," one American official here remarked.

The assassination of Issam Sartawi, the Palestine Liberation Organization's European representative, in Portugal eight days ago by a Palestinian extremist group already had raised fears of a new outbreak of terrorism in the Middle East.

The attack today was the most dramatic on a U.S. mission since embassies in Iran, Pakistan and Libya were assaulted in a one-month period in late 1979.

Militant youths seized the Tehran Embassy on Nov. 4 of that year and demanded that Washington send the deposed shah back to Iran for trial. The takeover triggered the 444-day hostage crisis.

Seventeen days later, a mob set fire to the U.S. Embassy in Islamabad in an attack in which two Americans were killed. The demonstrators apparently had been spurred by rumors that the United States was behind the seizure of the Grand Mosque in Mecca, Saudi Arabia, Islam's holiest city.

On Dec. 2 a crowd chanting pro-Iranian slogans broke into the U.S. Embassy in Tripoli and set fire to furniture. Nobody was hurt.

It seems a paradox that the U.S. Embassy in Beirut, after escaping serious damage throughout the 1975-76 Lebanese civil war and the Israeli siege of the capital last summer, was destroyed now with 1,200 American Marines in the city as part of the international peace-keeping force.

The heavily guarded, seven-story building was hit on several occasions by long-range rocket fire but never was the victim of a bomb attack despite the presence until last fall of a myriad of armed Palestinian and leftist Lebanese groups opposed to U.S. policy in the Middle East.

The Palestine Liberation Organization at times even provided active protection to the embassy and helped to free several U.S. officials seized by leftist factions during the civil war.

While the embassy itself has escaped major attacks, however, U.S. ambassadors have not. In June 1976 unidentified gunmen killed ambassador Francis E. Meloy Jr. and the embassy economic counselor, Robert O. Waring. The two diplomats, together with their driver, were dragged from their car and shot while on their way to a meeting with then-president Elias Sarkis.

The identity of the killers was never established, but the investigation pointed to the involvement of a Palestinian and Lebanese terrorist group with ties to Libya. The group was believed to have carried out the action in hope of sidetracking U.S. efforts to halt the then-raging civil war.

There also was an attempt on the life of U.S. ambassador John Gunther Dean in August 1980 during a concerted terrorist campaign to drive diplomatic missions out of Beirut. Gunmen ambushed Dean's bulletproof limousine as he was leaving the U.S. residency at Yarze in the heights above Beirut. A gunbattle between his bodyguards and the gunmen ensued, but the ambassador emerged unscathed.

The terrorist campaign, which included the assassination of the French ambassador, culminated in the destruction of the Iraqi Embassy in December 1981 in a car-bomb attack that left more than 60 dead and between 70 and 80 injured.

The terrorists responsible for the Iraqi Embassy bombing were never caught, and no group took credit for it. But they were widely believed to be acting on behalf of Iran or Syria, both bitter rivals of Iraq.