Washington — Moderate Arab regimes are nervous that the massive Israeli invasion of Lebanon, and the minimal Arab response to date, may provoke a violent popular backlash throughout the Arab world that could threaten many governments, both moderate and radical.
Arab diplomats here fear this is even more likely if Israel resumes its massive assault on western Beirut and attempts to gain total victory over Palestine Liberation Organization (PLO) forces holding out there.
''This will be destabilizing for the whole Middle East,'' says Ambassador Abdul Hadi Majali of Jordan. ''It could sweep out all the Arab regimes.''
Arab leaders also fear PLO remnants may splinter into terrorist groups that will target their regimes.
This would upset predictions by some Reagan administration officials that the Israeli invasion will open up possibilities not only for a stable Lebanon but for an era of peace throughout the Middle East.
Their failure to respond to the Israeli invasion seems to have stunned the Arabs themselves. Syria, which had 25,000 troops in Lebanon serving as an Arab-designated peacekeeping force, was the only country to respond militarily, but it entered into a cease-fire with Israel while the PLO was being decimated.
Iran has sent an estimated 1,000 ill-trained, poorly armed men to assist in the fighting in Lebanon.
But it was not until June 26, nearly three weeks after the invasion, that Arab foreign ministers gathered in Tunis. They then failed to produce a united response.
This painfully reminds the Arabs of their current state of division and disarray: Egypt neutralized militarily by its peace treaty with Israel, Iraq still embroiled in war with Iran, Syria rent by internal dissention, and the oil states preoccupied with the spread of radical Islamic fundamentalism from the Khomeini revolution.
Many Arab regimes are not displeased at the PLO's military demise. But the enormity of the Israeli attack on Lebanon, the first Israeli entry into an Arab capital, the decimation of Beirut, which has special economic and psychological ties to the Arab world, and the massive Lebanese and Palestinian civilian casualties have embarrassed and humiliated Arab leaders in their own eyes, and, they fear, in the eyes of their populations.
''It's even worse than after 1948 and 1967,'' says an Arab diplomat, referring to the tumultuous aftermath in the Arab world to the Arab-Israeli wars of those years. ''At least then the Arabs did something, but now they are doing nothing at all.''
No huge crowds have taken to the streets in Arab cities. Arab diplomats here attribute this to shock, the speed of the Israeli advance, and the initial lack - or control - of information presented in the Arab media. Reaction has also been slow because the outcome of the Israeli invasion is not yet fully clear.
''The changes right now are too great to comprehend,'' says an Algerian diplomat. ''But there will be an explosion later.''
At a minimum, Arab diplomats expect a period of intense soul-searching in the Arab world.
The intensity of popular reaction, Arab diplomats say, will depend heavily on whether Israel strives to totally defeat the PLO or agrees to a face-saving formula.
''It is important that Israel understand that they will not wipe out the Palestine national movement,'' says Egypt's Ambassador Ashraf Ghurbal. ''The lesson of the Middle East is that wars which end in victor and vanquished lead to another war. They resurrect feelings of vengeance and honor. But wars like (the Arab-Israeli October war of) 1973, which end in no humiliations, create opportunities for change.''
However, Arab diplomats see several major shifts in Middle East patterns that could evolve from a total Arab humuliation:
* An end to the moderate tendencies of the last decade that saw several Arab states groping toward a formula of mutual recognition with Israel and Egypt actually entering into a peace treaty. The Egyptians are especially upset at this prospect.
* A wave of anti-Americanism perhaps leading to closer relations between moderate Arab states and the Soviet Union. While Arabs were angered at the lack of response of both the US and Soviet Union, the Soviets were seen as passive while the US was viewed as actively abetting Israel.
* Further fragmentation in Lebanon, where any new government would be viewed as the product of an Israeli ''dictat.''
* A growing wave of radicalism in the Middle East from both Islamic fundamentalists and the radical left.