Saturday's fighting started when Hamas security forces moved into a Gaza City neighborhood to arrest members of the Hilles clan who were apparently suspects in a July 25 bombing in Gaza that killed seven people.
The crackdown sparked intense fighting with the pro-Fatah Hilles and caused many members of the heavily armed clan to escape to Israel.
By Sunday afternoon, at least 30 members of the Hilles clan were returned to Gaza by Israel and arrested immediately by Hamas. Hamas spokesman Ihab Ghousin said that those arrested who did not participate in any crime would be released while the others would stand trial. He added that the Shejaia neighborhood, where the clan lives, would be under Hamas control for the next three days as it is searched for explosives and weapons.
Hamas critics say the crackdown on Shejaia was part of a broader power play by the Islamists to shore up their power in the coastal territory of 1.4 million people as popular support for Hamas is waning.
"Hamas is trying to exercise power. They believe in their merit to have power and to exercise it and they don't want opponents – especially violent opponents. In this case of course, they have come across the strongest family perhaps in Gaza. At the end of the day, everybody – we Palestinians – have lost, but Hamas has won," says Eyad Sarraj, an independent politician and the Gaza commissioner of the Palestinian Independent Human Rights Committee.
Over the past week, spokesmen for the rival parties stepped up their rhetorical attacks on one another, with Hamas making open threats to repeat their Gaza takeover in the West Bank. The acrimony is complicating recent attempts by Arab mediators to convene Hamas-Fatah talks aimed at reconciling the 13-month rift.
"It will continue to escalate. Maybe they are negotiating by arresting each other before the real talks," says Hissam Jaberi, a Gaza-based reporter for the Al-Ayyam newspaper. "As long as we don't have a national program accepted by all the parties, we will continue to fight."
Hamas arrests on Friday included prominent members of Fatah's executive committee and Fatah's regional governors who Hamas had allowed to remain in office ever since seizing power in the Gaza Strip in June 2007.
For several days, Hamas has blocked the distribution of the three leading Palestinian daily newspapers in the Gaza Strip, accusing the broadsheets of being biased toward Fatah.
Despite the escalation in Gaza on Friday, Palestinian Authority (PA) security forces released several Hamas figures from custody, complying with a directive the previous day by President Mahmoud Abbas.
On Sunday, at Israel's Barzilai Hospital, six miles north of the Gaza border, 11 Fatah fighters were being treated for wounds received in Saturday's round of fighting. At least 100 Palestinians fled Gaza to the nearby Israeli crossing point Saturday after the Hamas crackdown on the Fatah stronghold in Gaza.
Under the guard of Israeli soldiers with M-16 rifles, some recounted coming under rocket and mortar fire from Hamas, and their 300-yard dash to the Israeli border with Hamas gunfire at their backs.
"I never thought I would live," says Midhat Shashar, a former PA security officer as he shifted uncomfortably in his hospital bed. "Hamas is in total control of Gaza.... There's no chance for Fatah to resist. They're all in jail."
The wounded man explained that Hamas officials had approached the Hilles family and asked them to surrender members of the family whom the Hamas had accused of planting the July 25 car bomb that killed five Hamas activists and a girl. The family denies the charges, accusing Hamas of shifting for their own internal power struggle.
"I don't consider them a government. I consider them a mafia," says Mr. Shashar as a delegation of Fatah officials filed in and out of the hospital rooms to shake hands with the wounded.
"We asked not to be sent back to Gaza. If we go back, they'll kill us," he says.
• Safwat al-Khalout contributed reporting from Gaza City.