At least three people have been killed and 10 injured in heavy fighting in Gaza as rival Palestinian factions challenge the authority of Hamas.
Mohammed Susi died after being shot in the head during the fighting
Members of the Islamist militant group were involved in fierce gun battles in Rafah against Islamic Jihad and in Gaza City against a clan allied to Fatah.
The dead included a 13-year-old boy caught in the crossfire, medics said.
Seven people have been killed since Thursday in the worst clashes since Hamas seized control of Gaza in June.
The violence erupted last week when the Hamas Executive Force tried to arrest members of the Heles clan, which is allied to Palestinian Authority President Mahmoud Abbas's Fatah movement.
The BBC's Aleem Maqbool says Hamas has been trying to exert its authority on small clans in Gaza and to suppress the political and militant opposition by Fatah supporters.
However, the tension and animosity towards Hamas from these groups has been growing and intensified by the isolation of Gaza by the outside world, our correspondent says.
Saturday's violence started when members the Heles clan fired at a vehicle containing Hamas security officers in the Shijaiyah neighbourhood of Gaza City, Hamas officials said.
Witnesses said Hamas snipers took over rooftops, while the clan blocked roads into the district with rocks and mounds of sand.
The two sides fired mortars and rocket-propelled grenades at each other, sending clouds of grey smoke into the air.
Mohammed Susi, a 13-year-old boy, died after being shot in the head during the fighting, medical officials said. A member of the Heles family was also killed when a grenade went off prematurely.
A senior Hamas leader, Mahmoud Zahhar, said the group was determined to impose order and was setting an example.
"Every family should respond to the new administration. That is the will of the Palestinian people in the West Bank and Gaza... to be governed by Hamas," he said.
"We are going to make an end for this chaos."
Later, Hamas forces clashed with members of Islamic Jihad near the southern town of Rafah. A 51-year-old woman died after being caught in the crossfire, medical workers said.
In other violence, an Israeli warplane fired a missile at a boat carrying two suspected Islamic Jihad militants, the Israeli military said. Hamas said they had been fishermen.