Syria used tanks and special commando units this weekend to bring under its firm control the remnants of Yasser Arafat's Palestinian fighting force in eastern Lebanon's Bekaa Valley.

On several hours' notice, Syria brusquely ordered the PLO this weekend out of the area near both Lebanon's war-torn Chouf mountains and Syria's front lines with Israel. Three senior PLO officers who balked were arrested. Syrian forces then forced a 130-vehicle convoy of 1,500 guerrillas to an area 60 miles north of their former positions, leading them into a much smaller valley where the fighters discovered they were encircled by Syrian tanks on the mountain peaks above them.

Early this evening, Arafat and his aides, from their provisional headquarters in this northern Lebanese port city, 35 miles west of the valley where his guerrillas were trapped, were trying to get food and water to them.

"I hope that we can solve this problem with our brother Syrians," Arafat said. "It's a tough struggle. . . . As you see, they are sieging my forces. Squeezing them."

Frantically conferring by two-way radio with their fighters throughout the day, Arafat aides appeared bewildered by their sudden forced march out of the Bekaa. They were suspicious that it was linked to a coming agreement between the American-backed Lebanese government and Syrian-backed Druze rebels. But they did not seem sure.

What was apparent, however, was the humiliation and fear here. Arafat spoke of "dangers" to the trapped fighters as aides talked worriedly of the possibility of a "massacre." But when word reached here that Syria might release the Palestinians if they agreed to give up arms, the aides talked of honor and resistance.

Fighters in the valley, five miles north of the town of Hermel and barren of any trees or cover, were said here to have taken up combat positions this morning against the tanks on the hilltops. But officials here carefully stressed that no order had been issued to fight.

"I prefer to find some way short of fighting," Arafat said.

Concerned for months that the Syrians would eventually make a deal in Lebanon at their expense, PLO operatives behind Syrian lines had bragged openly about their ability to upset those plans.

In the late spring and early summer, as Syrian-backed rebels opposed to Arafat's leadership fought PLO loyalists in the Bekaa, spokesmen in this last secure stronghold of loyalists said they were escalating their guerrilla operations against Israeli forces, in part as a threat to draw Syria into a bigger war they knew Damascus did not want.

By most accounts, both Syria and its Lebanese Druze allies sought to keep the Arafat forces out of the Chouf battles, preferring instead to use the Syrian-backed Palestinian dissidents as reserve fighters.

But Arafat's fighters, according to his lieutenants, found "special ways" around Syrian and dissident checkpoints to the mountains to field at least a token force in the fight- ing.

On Thursday evening, during the height of negotiations involving Syrian, Saudi and American mediators, Syrian intelligence officers abruptly appeared in the offices of Arafat loyalists in the Bekaa demanding that they leave--fighters, doctors, lawyers, nurses, typists and supply clerks included. They told them to go to Hermel, 60 miles north, and leave their artillery and heavy weapons behind.

Some of the PLO officers refused and were taken into custody by the Syrians.

After intense negotiations, agreement was reached the next day for all to leave, for the officers to be released and for the PLO units to take their heavy weapons with them, according to accounts here by Arafat and his aides.

The Syrians on Friday gave them two hours to get out and the first units began heading north.

Aides here said the Syrians told the PLO forces they had to go because they were an irritant to Lebanese who lived in the Bekaa.

Syrian tanks were at the front and rear of the convoy as it moved up the road. As they traveled, there was more wrangling about whether the caravan, which included trucks, artillery and taxis transporting about 20 families of fighters, would be able to keep the heavy weapons, but the Syrians relented and the caravan moved forward.

When they got north of Hermel late last night, however, the caravan discovered that the road leading to Tripoli was blocked by Syrian tanks. Their Syrian escorts ordered them to take the left turn into the valley, where they found themselves surrounded. Three officers who protested were taken away by the Syrians.

There was anxiety here today as Arafat and his aides attempted to assess Syrian motives. Would the Syrians take the heavy weapons and vehicles and force the fighters and others to walk to Tripoli, was one question asked. Or would they keep them there and try to coerce them to join the ranks of PLO rebel forces.

Late this afternoon, Arafat confessed he did not know what the Syrians wanted.

"You can say we are facing some troubles," Arafat said. "It is not a picnic."