GAZA, July 26 (Reuters) - Hamas security forces in the Gaza Strip arrested 120 men aligned with the rival Fatah faction on Saturday after an explosion there killed five Hamas gunmen and a girl, the Islamist group's officials and Fatah said.

Friday's blast next to a car used by the armed wing of Hamas killed three gunmen and the girl, and two others later died of their wounds in hospital, Gaza's ruling Palestinian Islamist group and medical officials said.

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 Palestinian President Mahmoud Abbas's secular Fatah faction to seize control of the territory a year ago.

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.

Hamas's armed wing issued a statement blaming "members of the fugitive party" -- a derogatory term for Fatah -- for Friday night's blast at a major junction outside Gaza City. Fatah denied involvement and said it was part of Hamas infighting.

"We have information that some elements are planning to carry out bombings against the interests and leaders of Hamas in order to sow anarchy," said senior Hamas leader Khalil al-Hayya, whose son, himself a gunman, was wounded in the explosion.

"The Fatah movement has no link whatsovever with these internal disputes within Hamas," a statement by Fatah said.

"The claim that Fatah carried out these explosions aims to cover up the fact that there are disputes within Hamas," added a statement from Abbas's office.

Hamas called on Gaza residents to turn out in their masses for the funerals of the six, who will be buried following noon prayers on Saturday.

At least 19 other Palestinians were hurt, medical officials said, many of them passersby on their way to the beach.

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.

The ceasefire stoked occasional intra-Palestinian tensions as Hamas has sought to prevent other groups from firing rockets across the border into Israel. (Reporting by Nidal al-Mughrabi, additional reporting by Mohammed Assadi in Ramallah, Writing by Ori Lewis; Editing by Catherine Evans)

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