Palestine Liberation Organization rebels seized control of Chairman Yasser Arafat's main supply base in the Bekaa Valley in a battle early today, apparently consolidating control over positions held by dissidents near the tense front lines in eastern Lebanon.

Arafat aides accused Syria of aiding the mutineers, charging that Syrian tanks had shelled PLO loyalist positions during the four hours of fighting in the predawn darkness.

Arafat, who was meeting in Damascus with his group's Revolutionary Council to consider reforms to quell the revolt when the fighting broke out, left abruptly from the Syrian capital. He took a circuitous route, avoiding rebel positions, to his stronghold in the Palestinian refugee camps in the northern Lebanese coastal city of Tripoli, which appears likely to be his next base.

He was described by aides as being "very upset" at Syria. Later today Arafat sent urgent telegrams to leaders of some Arab states asking them to "stop the Syrian-Libyan aggression" against him. The Libyan leader, Col. Muammar Qaddafi, has claimed responsibility for aiding the revolt against Arafat.

Syria not only did not deny that it had helped the rebels but for the first time appeared to drop its pose of mediator in the seven weeks of mutiny within Fatah, the largest of the factions in the PLO. For the first time, the Syrian news agency SANA today carried the full text of a statement by PLO dissidents. They claimed to have captured eight Fatah positions in the Bekaa Valley region in eastern Lebanon.

This morning's battles appeared to give Syrian President Hafez Assad stronger control over positions in eastern Lebanon along the front lines with Israel. Arafat loyalists behind Syrian positions previously had threatened to draw Syria into war with Israel by conducting hit-and-run guerrilla operations in and out of Syrian lines.

In addition to being a devastating setback to Arafat, the events appear to have made it more likely that he will choose Tripoli as a base, bringing him back into Lebanon nearly a year after the evacuation from Beirut. As the mutiny against him has grown in recent weeks, Arafat quietly has been building up PLO operations in the Tripoli refugee camps--basing his spokesman and the PLO newspaper there and most recently opening an office of Wafa, the PLO news agency.

There were unconfirmed reports here tonight that top Arafat aides in Damascus were closing down offices and leaving for Tripoli. Arafat said earlier today that he would not return to the Syrian capital until Syrian troops stopped supporting rebels in eastern Lebanon. He was reported by aides to have sent an angry message to Assad protesting Syrian "practices."

Syrian troops are posted on the outskirts of the violent, palm-shaded Tripoli but in their long occupation in Lebanese territory it is a city they never conquered. They have been involved repeatedly in clashes with Moslem fundamentalist militias here that residents claim are manipulated by the PLO.

There have been several street battles and shootings there during the past week, including an ambush Monday in which seven Syrian soldiers reportedly were killed, but the source of the conflict has been murky.

The Christian Phalangist radio in Beirut said that masked gunmen in Tripoli stormed into the mosque at 3 a.m. today and fired at worshipers, killing six and wounding nine.

Arafat aides have claimed that their fighters have stayed out of the battles, confining themselves to the camps where several hundred armed fighters in at least a half dozen styles of uniform live.

Arafat's Fatah Revolutionary Council was meeting in Damascus to consider reforms to accommodate rebel demands when pro- and anti-Arafat guerrillas began fighting shortly after midnight.

Arafat's spokesman Ahmed Abdel Rahman said later in Tripoli that the rebels "opened fire from Syrian positions toward our forces . . . . To our surprise they then advanced with a number of Syrian tanks toward our positions . . . firing on our positions."

"Our men were careful not to return the tanks' fire so as not to give the opportunity for a clash with Syrian forces," the said. "But the tanks continued to shell us and advance . . . together with the armed men and took control of the area."

The fighting occurred in the town of Majdal Anjar, in the Masna region of the Bekaa Valley near the Beirut-to-Damascus highway, about three miles from the Syrian border.

United Press International reported the following:

Arafat's men continued to hold the key town of Shtawrah, headquarters of the Fatah guerrilla group for eastern Lebanon, relief workers said in Beirut. Official Beirut radio said the Arafat loyalists "retreated" from Majdal Anjar and Souairi along the highway, to Talbaya, a few miles deeper into Lebanon.

Lebanese police said earlier that Syrian tanks were deployed around the trouble spots with no indication they tried to stop the fighting.

The Damascus highway is under the ultimate control of the Syrian and Israeli armies, with the dividing point at Sofar, but Palestinian guerrilla units depend on the Syrian part as a supply route.