At least 11 Palestinians have been killed and seven wounded in street fighting between Hamas security forces and a powerful Gaza family, Palestinian security officials say.
Fighting broke out on Monday night when the forces went to arrest members of the Doghmush family who had been accused of killing a police officer in Gaza City.
At least one family member and one Hamas policeman were killed, officials said.
A street fight followed in Gaza City's Sabra neighbourhood, where the Doghmush family has most of its support, taking the toll to 11.
The Doghmush clan is one of the most powerful families in the Gaza Strip and is accused of having links with al-Qaeda. Family members kidnapped Alan Johnston, the BBC correspondent, and held him for almost four months in 2007.
The fighting is the deadliest in the territory and since Hamas clashed with Hellis, a pro-Fatah clan, in July and August.
Relations between Hamas and Fatah forces loyal to Mahmoud Abbas, the Palestinian leader, have been tense since Hamas took power in the Gaza Strip in June 2007.