AP quoted witnesses as saying the truck carried homemade weapons, and Palestinian security officials as saying that the blast apparently was caused by the mishandling of explosives.

Hamas blamed Israel, saying Israeli aircraft flew overhead during the rally. Israel denied it was involved.

Witnesses said participants crowded around the pickup truck carrying fighters when the explosion went off. The witnesses added that the truck carried homemade weapons, but did not know whether they were rockets or grenade launchers.

Earlier on Friday, Israeli soldiers killed three Palestinian fighters in Tulkarim in the West Bank, drawing strong condemnation from another Palestinian resistance group, Islamic Jihad.

The three fighters from al-Quds Brigades, the military arm of Islamic Jihad, were killed just hours after Israeli soldiers evacuated an army base.

Afterwards, Aljazeera reported  that nine rockets were fired from Gaza towards the Israeli township of Sderot, in the first such incident since Israeli forces completed a pullout from the territory 10 days ago. No casualties or damage were reported from the rocket.

Israeli Military radio confirmed only one rocket impact. "Other explosions were heard in the region but no (other impact in Israel) has been confirmed," a source said.

Khalid al-Batsh, a spokesman for Islamic Jihad, held Israeli Prime Minister Ariel Sharon and Defence Minister Saul Mofaz responsible for what he called "a deliberate assassination".

Also on Friday, Palestinian police and Hamas fighters briefly exchanged fire, leaving an armed Hamas man wounded, security officials and hospital doctors said.

At talks between Palestinian leader Mahmud Abbas and Gaza representatives of the main factions on Wednesday, fighters agreed to stop by Saturday all armed rallies and marches celebrating Israel's pullout from Gaza.