Hamas fighters captured one of the last bastions of forces loyal to Western-backed Palestinian President Mahmoud Abbas on Thursday and the group declared the "liberation" of the Gaza Strip.
Green Hamas flags fluttered from the rooftop of the Preventive Security headquarters in Gaza City, a potent sign that Hamas had taken charge after six days of bloodshed in which more than 100 gunmen and civilians have been killed.
At least 20 Palestinians were killed across Gaza on Thursday, hospital officials said, including 18 Fatah men whose bodies were found in the Preventive Security compound. Others were seen led away, bare-chested and facing an uncertain fate.
As resistance turned to rout and Hamas men rampaged through Fatah offices in scenes reminiscent of the fall of besieged cities down the ages, the White House accused the group of "acts of terror ... against the Palestinian people".
The fighting brought closer a division between an Islamist-controlled Gaza Strip and a West Bank where Abbas's Fatah faction holds sway, and took Palestinians further from their dream of an independent state in both areas.
Abbas, who unlike Hamas has embraced peaceful negotiation with Israel, was set to formalise the split by dismissing a three-month-old unity government with Hamas and putting himself in charge of an emergency cabinet, aides said.
For Hamas fighters, in their camouflage uniforms, the fall of the security headquarters was a cause for celebration. They fired gunshots in the air and handed out chocolates to local people in the coastal enclave.
"What happened today in the Preventive Security headquarters was the second liberation of the Gaza Strip, this time from the herds of collaborators," Hamas official Sami Abu Zuhri declared. The first was Israel's 2005 pullout of troops and Jewish settlers from Gaza.
Stripped to the waist, several defenders, their hands raised in surrender, were herded out of the compound by their Hamas captors. Hamas officials said fighters had orders to kill certain Fatah leaders and there were unconfirmed reports, as in previous days, of prisoners being shot. Hamas continued to pound Abbas's presidential compound in Gaza City and another Fatah stronghold with mortars, though both remained under Fatah control.
The Islamist group said it had swept up other Fatah strongholds across Gaza, including a security office in the southern town of Rafah on the Egyptian border. Pro-Fatah broadcasts in Gaza went off the air and the Voice of Palestine radio station was set ablaze.
VIOLENCE SPREADS TO WEST BANK: Some Fatah gunmen retaliated in the West Bank, shooting and wounding a Hamas man near Ramallah, and seizing Hamas gunmen in the towns of Jenin and in Nablus, where they stormed a Hamas office and hurled its computers out the window.
Abbas was poised, aides said, to dissolve the Hamas-led "unity government" that Fatah joined in March under a Saudi-brokered deal aimed at ending internal violence and easing Western sanctions imposed for Hamas's refusal to recognise Israel.
A senior official close to Abbas told Reuters he wanted to head an emergency cabinet himself, a move that could effectively divide control over the two Palestinian territories.
Faced with the prospect of a Hamas-run Gaza Strip, Israeli Prime Minister Ehud Olmert has said an international force along the territory's border with Egypt should be considered.
Israeli political commentators have already dubbed the Gaza Strip "Hamastan", though they played down the prospects of an Israeli military invasion to confront Hamas. "We have to find other options to this, however tempting," Amos Gilad, a senior security official, said on Israel Radio.
The European Commission suspended humanitarian aid to Gaza. "The humanitarian situation is catastrophic, we have had to withdraw our operators," European Aid and Development Commissioner Louis Michel told Reuters in Brussels.