GAZA CITY (AFP) - Hamas-run security forces fanned out across Gaza City Saturday, clashing with rival gunmen and arresting dozens of people after a bomb blast killed five senior Palestinian militants and a girl of five. The explosion late on Friday near a beach outside Gaza City was the deadliest incident in weeks in the impoverished territory which has been ruled by the Islamist Hamas movement for more than a year. The cause of the explosion was not immediately known but Hamas blamed Palestinian president Mahmud Abbas's rival Fatah movement, accusing it of collaborating with Israel to undermine the Islamist movement. "The Fatah movement is behind this reprehensible crime," senior Hamas leader Khalil al-Haya told a crowd of thousands of supporters and other senior leaders at a funeral for those killed. "Those who carried out this crime are making war on God, on the security of Gaza, and on the resistance," he said. "They will not be released after six months but will be hanged from the gallows and shot." Thousands of supporters marched through the streets on their way to the funeral, carrying the bodies on stretchers draped with green Hamas flags, firing guns into the air and vowing revenge. "It is up to Fatah to determine their position straightaway, if they are with these criminals, or if they are with the ranks of the Palestinian street in confronting the enemy (Israel)," Hamas chief Mahmud al-Zahar said. "This is a crossroads at which everyone must determine their position. We will not permit this scene to repeat itself," he told reporters at the cemetery, adding that police had information on who was behind the attack. In the hours after the attack Hamas-run security forces arrested dozens of people across the territory, mostly Fatah supporters, and exchanged gunfire and rocket-propelled grenades with a clan in Gaza's Tel al-Hawa district. A senior Fatah official who asked not to be named said security forces had arrested over 120 Fatah members and raided at least 41 party offices and associated organisations across Gaza, confiscating computers and documents. In a statement released by Abbas's office Fatah denied any involvement in the "mysterious explosion" and accused elements within the armed wing of Hamas of planting the explosives as part of an internal conflict.