The suicide bomb attacks on the U.S. and French peace-keeping forces here have had a devastating psychological impact on even war-hardened Lebanese, who are now wondering what spectacular horrors await them in the coming weeks and months.

Numbed by the enormity of the explosions and destruction, the biggest seen here since the bloody 1975-76 civil war, Lebanese are searching in their bewilderment for the message and the mastermind behind the attacks.

Some believe it was done mainly to humiliate and chase out the 5,800-man multinational force. Others speculate that it was aimed at undermining President Amin Gemayel on the eve of the National Reconciliation Conference in Geneva, and still others argue that the objective was to blow up the talks even before they begin next Monday.

At the public level, Lebanese feel events have suddenly spun out of their control and are wondering who can save them from total chaos if the multinational peace-keeping force, with an armada of warships just offshore, cannot even protect itself.

"The psychological effect of this is so enormous. It is like it happened in Zimbabwe," remarked Samir Franjieh, an intellectual renegade from the Maronite Christian Franjieh clan of northern Lebanon. "The question of Lebanon is for once the question of the whole Middle East. . . . Everyone is afraid of what the Americans might do."

For many typical Lebanese, the fact that a bomb-laden truck could penetrate the supposedly formidable defenses of the main Marine compound is at once incomprehensible and terrifying.

"How can they protect us if they cannot even protect themselves?" remarked the badly shaken secretary of a high-ranking Lebanese political figure. "It's worse then horrible," she said, seeking in vain for the word to describe the despair that has seized many Lebanese now.

In the aftermath of the bombings and the fingering by U.S. officials of Shiite Moslems allied with Iran and aided by Syria as the probable culprits, the mood of the Shiite population of Beirut's southern suburbs has turned to one of fright, indignation and defiance. It is a highly explosive combination that could easily touch off a major confrontation with the tense and bitter marines at the nearby airport.

"If the Americans are here to make peace, welcome," one local Shiite militia leader said. "If they are going to fight, it is going to be another Vietnam for them."

"Every shell that falls now on Burj al Barajinah," remarked a resident of the Shiite neighborhood nearest the airport, "they say it is 'the Marines' no matter where the shooting comes from."

Lebanese are divided whether the bombings have further sapped their already weakened president or could, on the contrary, play into his hands in his bid to obtain stronger American backing.

One respected Christian commentator of a leading daily here, who asked to remain anonymous, said the devastating attacks on the American and French soldiers had laid bare the thinness of Gemayel's power base and the fragility of what remains of the Lebanese state here in Beirut.

"It's a state that no longer officially exists," he remarked. "There is a vacuum at the presidency."

The only Lebanese faction that seems to believe there are some potential "beneficial" effects to be drawn from the disaster are the Christian Phalangists. They see the American blaming of Syria and Iran as a confirmation of one of their favorite theses--that the woes of Lebanon are entirely the fault of outsiders, whose forces must be removed from Lebanese soil before any real national reconciliation is possible.

The Phalangists are drawing hope from the initial firm American and French reaction to the bombings that both nations will now retaliate against the Syrians and their local allies, thereby strengthening the Phalangists' weakened military and political position and that of the central government.

Asked whether he thought the disaster had undermined Gemayel, as many Christians and Moslems in west Beirut feel, one member of the Phalangist party political bureau replied, "On the contrary, it will harden the American position toward Syria. This strengthens Amin's position."

Another member, Alfred Maady, said he thought it all depended on whether the Americans and French will in fact retaliate to show they are ready and willing to use force--a determination still widely doubted in many quarters here.

"If there is no reaction and nothing happens, I can predict more attacks against the American presence here and the whole thing degenerating, Beirut going back to a terrorist center," he said.

Thus, the Phalangists, like the Shiite Amal militia, the Druze opposition and the Gemayel government, are all watching closely for different reasons to see what U.S. and French intentions are.

Whatever the two governments decide to do, their actions, or lack of action, seem certain to have far-reaching repercussions on the Lebanese political landscape and to shape the course of events here over the coming months.