Iran's Islamic revolution, at first hailed as a great victory for the Palestinian cause, is becoming a source of embarrassment for the mainline leadership of the Palestine Liberation Organization.

"Strategically, the changes in Iran have been very important for us," said one PLO official." The trouble is that there are some real nut cases over there."

The Tehran connection has proved particularly troubling because wrath stirred up in American public opinion by Iran's Islamic extremism tends to fall on the plo as well, Palestinians here say. This is happening just as the Palestinian case seemed to be getting a more favorable hearing in the United States, they add.

The result is at least a temporary setback for the improved PLO image cultivated by Yasser Arafat in recent months through visits to Western Europe, shows of moderation and a so far unsuccessful attempt to mediate with Ayatollah Ruhollah Khomeini for release of American hostages in Tehran.

Closer to home, the loudly trumpeted determination of a Tehran mullah, Mohammed Montazeri, to lead a band of Iranian volunteers to south Lebanon to fight alongside PLO guerrillas has added another irritant to already strained relations with the Lebanese government.

To keep the volunteers out, President Elias Sarkis has closed Lebanese airspace to Iranian planes and ordered consulates abroad to refer Iranian visa requests to Beirut. Government sources say Sarkis also has obtained assurances from Damascus that the Iranians will be barred from entering Lebanon overland from Syria.

An Iranian who described himself as the first volunteer to arrive nevertheless told a news conference in Beirut today that a contingent of several hundred from among 10,000 volunteers would be on the way this week-end in the name of the slogan, "Today, Iran. Tomorrow, Palestine."

The self-proclaimed volunteer guerilla refused to say how he entered Lebanon despite the government's precautions. Lebanese sources speculated that radical Palestinians acting on their own could have smuggled him across the border from Syria.

Many within the PLO regard the Iranian revolution as a step in the right direction and hail the tactics of Iran's Islamic militants as a slap in the face of "American imperialism."

Within the leadership offices of Beirut, though, even leftist PLO leaders draw back from the uproar over Iranian volunteers and insist that nobody invited them.

Palestinian military officials say the last thing they need in south Lebanon is a group of Iranians with no known military training, no ability to speak Arabic and doubtful willingness to follow orders from PLO commanders. s

"the thing is embarrassing," said an official of the Democratic Front for the Liberation of Palestine, a Marxist group to the left of but loyal to Arafat's overall PLO leadership. "It is just propaganda for that guy in Iran. We don't need anybody in the south. It is not men we lack there." i

Even if the Iranians could be useful, their presence in Palestinian positions in southern Lebanon would bring new troubles to the guerrilla leadership. Israeli Defense Minister Ezer Weizman has warned that his forces would be likely to react if the Tehran brigade showed up. Israeli jets have been making increased reconnaissance flights over the border hills in recent days.

Palestinian analysts fear that one outcome of the Tehran hostage drama will be a hardening of U.S. policy in the Middle East, with reduced sympathy for Palestinian concerns in the Egyptian-Israeli negotiations.

In addition, any U.S. military actions to free the hostages or retaliate for their capture would be likely to poison relations between the United States and the Arab world, again reducing any U.S. sympathy for Palestinian demands in the autonomy talks.

Despite the drawbacks, however, there is broad agreement within the PLO that the Iranian upheaval can benefit Palestinians in the long run. Officials point to the cutoff in oil deliveries to Israel and, more importantly, the general setback for U.S. interests in the Persian Gulf and the unsettling fallout on conservative U.S. allies in the area such as Saudi Arabia.

Even PLO moderates say this is part of a trend that could produce Persian Gulf regimes more strongly committed to using their wealth to defeat Israel. In addition, it coincides with the revolutionary bent of much Palestinian thinking, at least that done in public.

Many PLO offices these days have a poster on the wall entitled, "A List the People Will Complete." Red checkmarks identify as successes the political changes in Ethiopia, Vietnam and Spain. The last addition with a checkmark is Iran. Two more names are still without the red mark of success: cEgypt and Palestine.