The Palestine Liberation Organization voted tonight to join Syria and South Yemen in boycotting Tuesday's Arab summit conference, effectively dooming the conference to failure before it could open. c

With Arab ruler, presidents and prime ministers already arriving here to the boom of official salutes today, the PLO took its decision at a heated, four-hour meeting in Damacus of its 50-member Executive Council under the chairmanship of PLO chief Yasser Arafat.

In deciding "with great sorrow" to boycott the summit, the PLO was following the lead -- and pressure -- of Syria, its protector and ally in the five-member Steadfastness Front grouping the most adamant opponents of Egypt's Camp David settlement with Israel.

Syria had campaigned vigorously for the summit's postponement because of acute divisions among Arabs in the wake of the Persian Gulf war between Iraq and Iran. Last week, when a preparatory conference of Arab foreign ministers refused to comply with Syria's demands, it decided to boycott the summit and urged its hard-line allies -- South Yemen, Libya, Algeria and the PLO -- to do likewise.

South Yemen complied immediately, as has now the PLO, despite reservations among many of its members. The Algerians announced tonight that they also will stay away and Libya, which has announced a merger with Syria, is expected to join its allies in the front.

Since the main aim of the Arab summit was to formulate a unified Arab political and economic strategy for confronting Isreal and its supporters abroad in the future, the absence from the summit of Israel's most implacable enemies has for all intents and purposes made its deliberations irrelevant.

"The summit is meaningless without those two," said one Arab diplomat here. "There's no way any political decisions taken about Israel will have any relevance. All the summit is left with is a discussion of its plans for a greater economic integration and coordination in the Arab world and the usual reallocation of funds and subsidies from the Arab haves to the Arab have-nots."

Any hopes of presenting a united Arab front toward Israel and the world at large died last September when Iraqi President Saddam Hussein launched his army against Ayatollah Ruhollah Khomeini's anarchic Islamic Republic of Iran.

Although his army has proved far less successful than expected in the field, Saddan Hussein, ambitious and eager to become a dominant political force in the Arab world, has forged a powerful and surprisingly moderate alliance with such onetime adversaries as King Hussein of Jordan and King Khalid of Saudi Arabia.

Opposed to this bloc has been the Steadfastness Front, led by Syrian President Hafez Assad, a bitter personal rival of Saddam Hussein, who threw his support behind the revolutionary Iranians despite the fact they were non-Arabs engaged in a war against fellow Arabs.

Jealous of Saddam Hussein's growing stature in the Arab world, Assad has done everything possible to scuttle the summit lest its majority undermine the hard-line confrontationist policies followed by Syria and its allies.

Assad argued that no summit could succeed so long as the divisions, remained as sharp as they were. Instead, he tried to convince Arab leaders that there should be a series of smaller meetings to iron out differences before the main summit.

Caught in the middle was Arafat's PLO, which depends on Assad for its physical existence in Syria and Lebanon as much as it depends on the magnanimity of subsidies from such oil-rich, conservative states as Saudi Arabia, Kuwait and the United Arab Emirates.

For the last 10 days Arafat has scurried about from one Arab capital to another, pleading for at least a two-week, face-saving postponement of the summit so some semblance of unity could be created. At the same time, Palestinian sources said, he quietly sought to gain understanding for the reasons why he might be forced by Syria to boycott the conference and to seek guarantees that the $3 million-a-year subsidy he gets from the Arabs would be increased as previously planned.