Hamas leaders blamed Fatah for Friday's explosion, though the circumstances remain unclear. Fatah leaders denied involvement, and an aide to Palestinian President Mahmoud Abbas said the Hamas crackdown on Fatah - the toughest in recent months - is reducing prospects for eventual reconciliation.

If Hamas was targeted, it would be the deadliest such attack since the Islamic militants ousted Fatah-allied security forces from Gaza in a violent takeover more than a year ago.

The blast went off underneath a car parked near a crowded Gaza City beach on Friday evening, Hamas officials said. At the time, a group of Hamas activists was picnicking on the beach, Hamas said. The blast shattered the windows of the white car and damaged its doors.

Three Hamas members and the girl were killed immediately. Two more Hamas activists died of injuries on Saturday and 15 people were wounded, according to a statement by the Palestinian Health Ministry.

Among the dead were a senior Hamas field commander and Iyad al-Hayeh, a nephew of Hamas politician Khalil al-Hayeh, health officials said. Osama al-Hayeh, a son of the politician, was wounded.

Tensions were palpable on Gaza's streets following the blast. Thousands of Hamas loyalists marched in the funeral of the explosion's victims, waving their movement's green flag. Gunmen fired automatic rifles into the air and angrily vowed revenge.

"The collaborators who did this must be hung in Palestine Square and shot," al-Hayeh, the politician, said in a eulogy, referring to Gaza City's main square.

"The people who stand behind this (attack) are from Fatah," al-Hayeh said. "We hold their leaders responsible....and say, either be nationalists or collaborators, the choice is yours," he said. Al-Hayeh had previously survived two Israeli assassination attempts that killed members of his extended family.