Rebels opposed to the leadership of Palestine Liberation Organization Chairman Yasser Arafat overran three loyalist military positions before daybreak today, killing at least 15 of Arafat's supporters and wounding 20 others.

The pro-Arafat forces issued a communique from its provisional headquarters in the northern port city of Tripoli reporting the loss of three bases in eastern Lebanon's Bekaa Valley and the casualty figures. The PLO news agency Wafa quoted a Palestinian military spokesman as saying that "since yesterday all our positions and forces in the Bekaa have been attacked" by the rebels and accused Syrian and Libyan forces of aiding the rebels by "ringing and pinning down our forces."

The PLO communique called the attacks "the first link in the massacre chain to liquidate" the mainstream Fatah guerrillas that had been predicted by Arafat last week.

The attacks in the Bekaa Valley continued into the evening, according to Wafa, and involved exchanges of fire across the busy Beirut-to-Damascus highway.

Today's attacks were the bloodiest fighting yet in the eight-week-old rebellion that was begun by guerrillas protesting Arafat's policies and seeking a tougher military and political stance. Arafat has bitterly accused the Syrians of backing the rebels and supporting their attacks. The Syrians expelled him last week from Damascus and the Syrian-held areas of the Bekaa.

It was not immediately clear exactly which three bases the Arafat loyalists were conceding had been lost. According to some PLO officials, the artillery base at Rawda, near Shtawrah, on the Beirut-to-Damascus highway, had been overrun, but the Wafa dispatch said the pro-Arafat forces in Rawda had repelled the attack. Other PLO sources said the rebels also captured a security post and another position outside Shtawrah, which is about 12 miles from the Syrian border.

According to Wafa, the rebels took some prisoners, including Nasr Yusuf, commader of Fatah's elite Yarmouk brigade. The report also said the rebels were stopping cars and "seizing occupants" along the roads between Darzaynun and Taanayel and Darzaynun and Kafr Zabad.

PLO officials in Tripoli said the fighting spread over an 11-mile area from Shtawrah to areas near the ancient city of Baalbek.

On a drive along the Beirut-to-Damascus highway at midday, tense Syrian soldiers could be seen massing near Shtawrah as the sounds of exploding mortars and small arms fire were heard on the battlefields amid populated areas farther east. By mid-afternoon, streets were cleared completely and shops shut.

State-run Beirut radio quoted the Lebanese governor of the Bekaa, Diab Younes, as saying that there were many killed and wounded, particularly among civilians, and a lot of damage, Reuter reported.

PLO officials in Tripoli said Syrian tanks pounded positions of loyalists and blocked their movement but allowed mutineers to roam freely. Rebels included not only breakaway forces of Fatah, the largest faction in the PLO and previously Arafat's main base of support, but also guerrillas in the pro-Syrian Saiqa and the Libyan-backed Popular Front for the Liberation of Palestine--General Command.

In Damascus today, Nimr Saleh, one of the leaders of the mutiny, vowed to fight loyalists "without mercy." Saleh accused Arafat of being a "deviationist" and "traitor" because of his aborted talks with Jordan's King Hussein on President Reagan's Middle East peace plan. He called for a "special relationship" between the PLO and Syria.

Arafat, who has been traveling throughout the region and Europe seeking to bolster his support since the mutiny began, flew from Tunis Tuesday to Algiers, The Associated Press reported. He is expected to meet with President Chadli Bendjedid and Foreign Minister Ahmed Taleb Ibrahimi, who has just returned from Damascus.

The Syrian yoke around the PLO strengthens the position of Syrian President Hafez Assad in any future bargaining. Many Lebanese Christians, who have strongly opposed Arafat and PLO policies here, say that they are now worried about Arafat's fate and fear a switch by the PLO to more radical policies.

Government officials here say they are worried that the intra-PLO fighting and the split between Arafat and Syria have distracted the attention of the Arab world away from the issue of Lebanon, where the mutiny threatens to break a fragile troop withdrawal accord brokered by the United States.

Special U.S. envoy Philip C. Habib met with King Fahd in Saudi Arabia today in apparent efforts to get Syria to discuss withdrawal of its troops from Lebanon. Syria has refused to withdraw, and Israel says it will not withdraw until the Syrian and PLO forces are out of Lebanon.

Habib later flew to Cairo and was expected in Lebanon later this week.