Jordan's King Hussein today placed his armed forces on alert and sent one of his crack Bedouin-led army divisions to the border with neighboring Syria after that country massed an armored division near the frontier in a show of displeasure against an Arab summit meeting here.

The sudden military face-off across the bleak 190-mile border between Syria and Jordan came as a Syrian-led boycott undermined the Arab summit, shattering its hope of projecting an image of Arab unity.

The confrontation overshadowed the summit's final deliberations today and buried efforts for a last-minute mediated compromise that might have coaxed Syria and four other hard-line Arab League members to attend the summit in time for a face-saving gesture of unity against Israel before the conference closes Thursday.

Syrian President Hafez Assad, who had demanded that the summit be postponed because of deep divisions among various Arab nations over the Iranian-Iraqi war, was in no mood for compromise. He ordered the mobilization of his forces -- estimated by Arab and Western diplomatic sources to number 20,000 soldiers and 400 tanks -- even before the 15 leaders of the Arab League's 21 members gathered here early this week for their unity summit.

The diplomats generally interpreted Syria's move as an "act of saber rattling" to show its displeasure with the summit, rather than a preparation for actual war. With about 22,000 of Assad's troops mired in peacekeeping operations in war-torn Lebanon and a deep political crisis threatening him at home, analysts said here that any attempt to attack Jordan would be "foolhardy and catastrophic."

But the Syrian move sent a wave of anger and unease through the Arab leaders at the summit conference, according to delegation sources, and Israel issued a warning against any Syrian attack on Jordan.

Saudi Arabian Crown Prince Fahd, who had personally tried to persuade Assad and his allies in the so-called Steadfastness Front to attend the summit, was reported by friends to be very upset. The Steadfastness Front consists of Syria, Algeria, South Yemen, Libya and the Palestine Liberation Organization.

Summit leaaders had adjorned deliberations for 24 hours last night to allow for private consultations among themselves and telephone calls to Damascus about a possible compromise that would have shelved discussions of any political issues in favor of the items of economic coordination and development on the agenda. The leaders hoped that such a compromise might have lured Syria and its friends to the final session to put their imprimatur on the summit agreements.

Although foreign diplomats and Jordanian officials tried to belittle the Syrian military move, it brought back memories of 1970. Then, during a civil war between King Hussein's forces and Palestinian guerrillas, Syria massed a similar armored division along the border and attacked.

However, the attack failed because at the last moment, Assad, then the Syrian Air Force chief of staff, refused to send his planes into the air to provide Syrian tanks with cover, reportedly out of fear that Israel would intervene and shoot down the planes. Without air cover, Jordan quickly destroyed the Syrian tank force and sent the invaders reeling northward in retreat.

As in 1970, Israel warned today against any Syrian encroachment on Jordanian territory, saying that this would be considered a threat to Israel's own security. Prime Minister Menachem Begin, after an emergency meeting with the parliament's foreign affairs and security committee, said that Israel was "watching the situation closely and that if Syria attacked Jordan it would not stand idly by."

At the summit, meanwhile, the Arab leaders met in a final working session tonight and approved a series of economic development plans. The most important of the measures called for the creation of a multibillion-dollar Arab fund for regioal economic development.

The final session also approved a "Joint Arab Action Plan" to confront Israel. The plan, however, was little more than a reassertion of previous Arab stands against Israel.

Jordanian Information Minister Adnan Abu Odeh told reporters that the summit also had discussed the Iranian-iraqi war. The war was the root of the Syrian boycott of the summit, since Syria and its allies sided with Iran, a non-Arab neighbor, Iraq.

Abu Odeh said that the summit had "agreed to reassert Arab efforts to defend Arab rights," an indication that Iraqi President Saddam Hussein, who dominated the conference, had won support for his war from the rest of the conferees. Abu Odeh said that other subjects discussed included the situation in Lebanon and how the Arab world should deal with the new U.S. administration. He said without elaborating that the summit "had decided to continue dialogue with everyone."