Fifteen Palestinians were killed and 80 were wounded as Hamas fighters overran one of Fatah's most important security installations in the Gaza Strip on Thursday, and witnesses said the victors dragged vanquished gunmen from the building and shot them to death gangland-style in the street in front of their families.

The capture of the Preventive Security headquarters was a major step forward in Hamas's attempts to complete its takeover of all of Gaza. Hamas followed up that victory by demanding Fatah surrender another key security installation.

Hamas also overran the southern city of Rafah, the second of Gaza's four main towns to fall into the Islamic group's hands.

Later Thursday, an explosion rocked Gaza City, and smoke was seen rising from a security post. Fatah security officials said forces positioned at the post had redeployed elsewhere and blown it up as they left, rather than let Hamas take it over.

Earlier, Palestinian Authority Chairman Mahmoud Abbas of Fatah, for the first time in five days of fierce fighting, ordered his elite presidential guard to strike back. But his forces were crumbling fast under the onslaught by the better-armed and better-disciplined Islamic fighters.

A Hamas military victory in Gaza would split Palestinian territory into two, with the Islamic extremists controlling the coastal strip and Fatah ruling the West Bank. Israel was watching the carnage closely, concerned the clashes might spawn attacks on the southern border.

Defense Minister Amir Peretz told a weekly meeting of security officials that Israel would not allow the violence to spread into attacks on southern Israel, meeting participants said.

The battle for the Preventive Security complex brought the day's death toll to 25 by mid-afternoon, hospital and security officials said. About 90 people, most of them gunmen but including children and other civilians, have been killed since a spike in violence Sunday sent Gaza into civil war.

Fatah said Hamas shot to death seven of its fighters outside the Preventive Security building. A doctor at Shifa Hospital, who spoke on condition of anonymity for fear of reprisal, said he examined two bodies that had been shot in the head at close range.

A witness, who identified himself only as Amjad, said men were killed before their wives and children.

"They are executing them one by one," Amjad said in a telephone interview, declining to give his full name for fear of reprisals. "They are carrying one of them on their shoulders, putting him on a sand dune, turning him around and shooting."

As Hamas took this major battle spoil, the Palestine Liberation Organization's top body recommended that Abbas declare a state of emergency and dismantle Fatah's governing coalition with Hamas. Abbas said he would review the recommendations and make decision within hours, said an aide, Nabil Amr.

After the rout at the Preventive Security complex, some of the Hamas fighters kneeled down outside, touching their foreheads to the ground in prayer. Others led Fatah gunmen out of the building, some shirtless or in their underwear, holding their arms in the air. Several of the Fatah men flinched as the crack of gunfire split the air.

"We are telling our people that the past era has ended and will not return," Islam Shahawan, a Hamas spokesman, told Hamas radio. "The era of justice and Islamic rule have arrived."

Sami Abu Zuhri, another Hamas spokesman, heralded what he called "Gaza's second liberation," after the 2005 disengagement.

Gunmen and civilians were looting the compound, hauling out computers, documents, office equipment, furniture and TVs.

Hamas had been tightening its ring around the Preventive Security complex for three days, stepping up its assault late Wednesday, with a barrage of bullets, grenades, mortar rounds and land mines that continued until the compound fell. Electricity and telephone lines were cut, and roads leading to the complex were blocked. Hamas claimed it confiscated two cars filled with arms sent as reinforcements.

The Islamic group was also training its guns Thursday at three other key command centers in Gaza City.

In a broadcast on Hamas radio, the Islamic fighters demanded that Fatah surrender the National Security compound by mid-afternoon. Light clashes were taking place there when the ultimatum was delivered.

Rocket-propelled grenades were also being fired toward Abbas's Gaza compound, provoking return fire from his presidential guard. For the first time since the fighting began, Abbas ordered his guard to go on the offensive against Hamas at the compound, and not simply maintain a defensive posture, an aide said.

The intelligence service compound was under siege as well, with Hamas firing dozens of rocket-propelled grenades in its direction.

In Gaza's south, Hamas trounced Fatah in Rafah, taking over the Preventive Security building in that town. It was the second main Gaza city to fall to the Islamists, who captured nearby Khan Younis on Wednesday.

"I can see the Preventive Security building in front of me. Hamas has raised its green flags over it," a civilian resident, who identified himself only as Raed, said by telephone. "There are men carrying away equipment from inside. ... (The Fatah-allied) National Security men ran away."

Hospitals were operating without water, electricity and blood. Even holed up inside their homes, Gazans weren't able to escape fighting that turned apartment buildings into battlefields.

Moean Hammad, 34, said life had become a nightmare at his high-rise building near the Preventive Security headquarters, where Fatah forces on the rooftop were battling Hamas fighters.

"We spent our night in the hallway outside the apartment because the building came under cross-fire," Hammad said. "We haven't had electricity for two days, and all we can hear is shooting and powerful, earth-shaking explosions.

"The world is watching us dying and doing nothing to help. God help us, we feel like we are in a real-life horror movie," he said.

Shaher Hatoum, a nurse at nearby Al Quds Hospital, said the facility had no electricity, water or blood, and that wounded were propped up on ward floors. Hundreds of bullets flew through windows, and fighters ignored the hospital's appeals to hold fire just long enough to have the generator and water pipes fixed, Hatoum said.

"We are waiting here for our our end," Hatoum said.

In Syria, meanwhile, a senior Hamas official warned Fatah to keep the violence contained to Gaza.

"This is very dangerous for our people," Moussa Abu Marzouk said by telephone in Damascus, where several top Hamas leaders live in exile.