A top Palestinian guerrilla leader was killed last night in an ambush carried out behind Syrian lines in Lebanon's eastern Bekaa Valley, the Palestinian news agency Wafa said today.

Saad Sayel, better known as Abu Walid, was assassinated while traveling between Baalbek and Rayak, a major Syrian military staging area, after having completed an inspection tour of Palestinian positions, the agency said. It blamed "Zionist murderers and their criminal agents" but gave no more detail of who the assailants were.

Sayel, variously described as chief of staff or head of the Palestinian guerrilla army's "operation room," was the first high-ranking guerrilla known to have been killed since the Israeli invasion of Lebanon June 6. Palestine Liberation Organization Chairman Yasser Arafat was reported to have flown to Damascus from a pilgrimage in Saudi Arabia. Sayel is to be buried in the Syrian capital on Wednesday.

Reports here said 30 gunmen, armed with automatic rifles and rocket-propelled grenades, ambushed Sayel, shooting him in the abdomen, and he died later in a Damascus hospital. Observers noted that the Israelis have never before been reported to operate behind Syrian lines in the Bekaa, where many Palestinian guerrillas moved after being forced out of Beirut.

The fact that Wafa did not directly accuse Israel suggested it might not have carried out the killing. Other speculation here ranged from Christian militiamen acting on behalf of the Israelis to a dissident faction within the PLO or even Syrian agents.

Sayel was a key figure in the battle for West Beirut this summer, in charge of organizing defenses against the Israeli air, ground and sea assault. He was a top adviser to Arafat on military matters.

Among the last to leave West Beirut, he since had been responsible for organizing the 5,000 guerrillas based in northern and eastern Lebanon. He made frequent trips between the Bekaa and Syria and was reported to be a strong partisan of continuing the armed struggle against Israeli forces still in Lebanon.

Sayel was an officer in the Army of Jordan until Palestinian fighters were ousted by that country in 1970. He joined Arafat's Fatah movement and was credited with having turned the guerrillas into a more professional fighting force.