Paramilitaries in Hamas's self-styled police Executive Force armed with anti-tank missiles exchanged fire with members of a powerful Gaza clan, the majority of which supports political rivals Fatah, witnesses said.
Trouble broke out in Gaza City when the Executive Force stopped a Palestinian Authority vehicle carrying former police officer Adel Hellis.
Three Hellis relatives and an Executive Force officer were subsequently killed, said an official at the Hamas-run health ministry, Maawia Hassanin.
Another 30 people were wounded, including one critically, said medical officials, revising up an earlier toll of three dead and 22 wounded.
Witnesses said the Executive Force blew up Adel Hellis' home but that the former police officer, who stopped work after Hamas routed Fatah in a bloody takeover of the Gaza Strip four months ago, was not hurt.
The clashes lasted about eight hours but ended before daybreak under a truce mediated by armed group, the Popular Resistance Committees.
Most of the Hellis clan are loyal to Fatah, the secular political party of Palestinian president Mahmud Abbas. Its loyalists were routed by Hamas in the mid-June take over following months of political rivalry and sporadic violence.