(Recasts with Hamas storms Abbas stronghold, two senior Fatah officials held, details)
GAZA, July 26 (Reuters) - Hamas security forces stormed the office of a Palestinian news agency run by President Mahmoud Abbas on Saturday and arrested dozens from his Fatah faction in their biggest crackdown since seizing Gaza, Fatah sources said.
The sweep, in which 162 Fatah activists were arrested including two faction officials, came in response to an explosion on Friday that killed five members of the armed wing of the Gaza Strip's ruling Islamist Hamas group and a girl.
The blast, the third of its kind in a day, marked one of the biggest flare-ups in internal Gaza violence since Hamas routed the forces of Abbas's more secular Fatah faction to seize control of the coastal territory a year ago.
The sources said Hamas security men seized computers and files at the Gaza offices of the WAFA news agency, a Palestinian media outlet run by Abbas, and stormed 40 other Fatah offices.
Two of those arrested on Saturday were senior Fatah officials, including, Ahmed Naser, a political Fatah leader in Gaza, and Abu Al'Abed Khattab, a former major-general in the Palestinian Authority, the sources said.
A Palestinian said to have worked as a cameraman for a German television station was also among those held. But a security source said the man, Sawah Abu Saif, was arrested as a suspected Fatah activist and not for being a journalist.
Hamas blamed "members of the fugitive party" -- a derogatory term for Fatah -- for Friday night's blast at a major junction outside Gaza City.
Thousands turned out for the funerals of the six victims of Friday's attack, some chanting "Revenge, revenge" as shots were fired into the air.
Senior Hamas leader Khalil al-Hayya, whose nephew was killed in the blast and whose oldest son was wounded, vowed to punish those responsible.
"Those who did this must be hanged in a public square and must be fired upon," Hayya said before the burials.
Hamas leader Ismail Haniyeh convened an emergency cabinet meeting and in a statement afterwards ministers said the bombing was "proof that Fatah is not interested in resuming any dialogue".
Fatah officials in Ramallah denied Fatah had any link to the violence and blamed it on Hamas infighting. A statement from Abbas's office said: "The claim that Fatah carried out these explosions aims to cover up the fact that there are disputes within Hamas."
A group called the "Al-Awda Brigades", which said it is aligned with Fatah, claimed responsibility for the attack. The authenticity of the claim could not be verified.
"The turn will come to all those who shared in executing and liquidating our people," the al-Awda statement read. "Our revenge will reach all members of the black militias of the executive force and leaders of Qassam (Hamas)."
Abbas, finding his authority limited to the occupied West Bank, split with Hamas and revived peace efforts with Israel. He recently sought reconciliation with his Islamist rivals but they have balked at his precondition that they give up Gaza.
The factional violence has eclipsed Israeli-Palestinian fighting in Gaza, where an Egyptian-brokered truce has largely held since last month despite some violations on both sides. (Reporting by Nidal al-Mughrabi, additional reporting by Mohammed Assadi in Ramallah; Writing by Ori Lewis and Allyn Fisher-Ilan;)