Through hail and torrential rain, Palestinian commando leader Yasser Arafat marched in a martyr's funeral procession today for Said Hammami, his slain envoy to London.

Even by Palestinian standards - and the guerrillas have commemorated the loss of many a comrade over the years - the turnout in the muddy streets of the Sabra refugee camp was impressive.

Drenched and marching with Arafat and Hammami's widow, Khalia, were the principal lieutenants of the often rival factions making up the Palestine Liberation Organization - which is now formally united behind Arafat against Egypt's go-it-alone peace offensive with Israel.

Among the marchers was Zuhair Mohsin, leader of the pro-Syrian Saica movement who called Arafat a "spy and traitor" in Novemeber for not denouncing Egyptian President Anwar Sadat quickly enough.

Arafat, in a brief address, blamed American-controlled Zionism and imperialism" for the death of Hammami, 36, a moderate slain Wednesday in the PLO LOndon office by unknown assassins.

Although Hammammi's known willingness to meet with Israeli doves made him a potential target for Arab hard-liners. London investigators have discounted any conspiracy. The "amateur" methods of the assassin led police there to suspect he was a zealot acting alone.

West Bank Palestinians and Israelis who dealt with Hammami nevertheless have said that he may well have been a victim of his flexibility toward Israel.

Yesterday, Abu Iyad, Arafat's principal lieutenant and fovorite channel for frank talk, accused Iraqi-based Palestinian renegade Abu Nidal of masterminding the murder.

There were other suggestions that Hammami was killed because he was involved in current contacts with Israel, that his execution was designed to prevent reconciliation between Iraq and Syria or that it was meant as a direct warning to Arafat to abandon any efforts to bring about a negotiated peace.

Hammami's body, flown in from London last night, was taken late today to Amman, Jordan, where Hammami spent his childhood after his family fled from Palestine when the state of Israel was created in 1948.