Hamas's security forces launched a massive crackdown over the weekend on Fatah supporters and institutions in the Gaza Strip following an explosion that killed seven people, five of them members of Hamas's armed wing, Izzadin Kassam. Fatah representatives and human rights organizations said at least 160 men had been rounded up, while 40 Fatah-affiliated institutions were either raided or shut down. A Fatah-affiliated group called Al-Awdah claimed responsibility for the explosion that killed the five Hamas men. However, Fatah officials in Ramallah strongly denied responsibility and said the killings were part of a "settling of scores" among Hamas militiamen. The crackdown on Fatah is the largest of its kind since Hamas took full control of the Gaza Strip in June 2007. The explosion took place outside a cafeteria on the beach in Gaza City, where the Hamas militiamen were picnicking. Eyewitnesses said a bomb planted beneath their vehicle was detonated by remote control, killing all five gunmen and a seven-year-old girl identified as Sereen Safadi. The five men were identified as Ammar Musbeh, a prominent leader of Izzadin Kassam from the Shajayieh neighborhood of Gaza City, who had previously escaped several attempts on his life by the IDF; Iyad al-Hayah, a nephew of senior Hamas official Khalil al-Hayah; Nidal al-Mabid; Osama al-Hilu; and Hassan al-Hilu. At least 22 people were wounded by the explosion, including Munzer al-Ghamari, director of the Hamas-controlled Ministry of Wafk [Islamic Trust] Affairs, eyewitnesses reported. Earlier, a bomb went off outside the home of Marwan Abu Ras, a top Hamas official in the Gaza Strip. No one was hurt and no damage was reported. Hamas policemen arrested a Fatah activist on suspicion of involvement in the attack. In a third incident, a bomb went off outside Al-Jazeera Cafe in Gaza City, killing one person. Sources in the Hamas Ministry of Interior said the victim, who was not identified, was killed when he tried to plant the bomb outside the cafe. They said the assailant belonged to a radical Islamic group that had been targeting cafes, restaurants, hair salons and Christian institutions in the Gaza Strip over the past two years. Speaking at the funeral of the Hamas victims, Hayah accused Fatah and its "treacherous" leadership in Ramallah of being behind the killings. "We have no doubt that Fatah and the traitors sitting in Ramallah were behind this crime," he said. "We urge the [Hamas] security forces to take tough measures to punish the culprits and to ensure that they are executed." Thousands of Hamas supporters carrying the movement's green flags attended the funeral, which quickly turned into an anti-Fatah demonstration. Hamas militiamen fired AK-47 rifles into the air as others chanted slogans condemning Fatah and its leader, Palestinian Authority President Mahmoud Abbas, as "Zionist agents." The Hamas government, which held an emergency meeting Saturday to discuss the violence, accused Fatah of standing behind a "terrorist scheme targeting the mujahedeen [warriors] and their leaders." Vowing to avenge the death of the Hamas men, the government accused Abbas and his aides of "misleading" the Palestinian public by talking about the need for reconciliation with Hamas. Ahmed Abdel Rahman, a senior Fatah official and adviser to Abbas, condemned the killings as "irresponsible," saying Fatah opposed the use of violence. He added that he did not rule out the possibility that the explosion was the result of an internal dispute, noting that this would not the first time Hamas militiamen had been involved in such killings. Fatah officials claimed that the five Hamas men were known as supporters of Ahmed Ja'abari, commander of Izzadin Kassam, who is currently involved in a power struggle with Imad Aqel, a senior member the armed wing. The Fatah officials said Aqel's supporters have been trying recently to remove Ja'abari from the leadership of Izzadin Kassam. Ashraf Juma'ah, a Fatah legislator from the Gaza Strip, condemned the killings and said "Israeli agents" were responsible. The Gaza-based Palestinian Center for Human Rights said at least 160 Fatah members and supporters had been arrested by Hamas since Friday's explosion. Almost all the detainees were from Gaza City or the northern Strip, the center said. Ziad Abu Amr, an independent legislator who was foreign minister in the former Hamas-Fatah unity government, said Hamas policemen raided his office in Gaza City, and confiscated his car, documents and equipment. Hamas's security forces also stormed dozens of Fatah-affiliated institutions across the Gaza Strip, including many nongovernmental organizations that provide services to the Palestinian public. Among the institutions targeted by Hamas were some that help the handicapped, women, orphans, farmers, students and refugees.