A Palestinian militant from Hamas stands guard on a rooftop in Gaza City, Tuesday, June 12, 2007. Hamas gunmen captured the headquarters of the Fatah-allied security forces in northern Gaza, seizing control of a key prize in the bloody power struggle between the sides, Hamas and Fatah officials said. [AP]

In one dramatic victory, hundreds of members of a Fatah-allied clan that had fought fiercely surrendered to masked Hamas gunmen. They were led ¡ª arms raised in the air ¡ª to a nearby mosque.

Fatah fighters desperately tried to cling to their positions, but appeared outgunned by Hamas. One of the battles raged around the headquarters of the Fatah-allied Preventive Security, with both sides firing wildly from high-rise rooftops.

Violence between the factions, which nominally share power in the Palestinian government, has rapidly spiraled toward all-out civil war.

Dr. Wael Abdel Jawad, a physician trapped in his apartment in the line of fire, said he heard Fatah fighters shouting at colleagues on an adjacent roof to send them more ammunition.

"All of us are terrified here. Shooting came through the windows of our apartment, children are screaming. We are hearing from a nearby mosque the call by Hamas to surrender," he said.

President Mahmoud Abbas of Fatah called the fighting "madness," but his appeals for a cease-fire rang increasingly hollow as Hamas gunmen took over or destroyed one base or another of his security forces.

Hamas has ignored calls for a cease-fire, and its hard-liners said the offensive would continue.

The State Department denounced the violence as a direct attack by the most radical elements of Hamas on legitimate Palestinian authorities. Spokesman Sean McCormack said Washington had no indication that Israel might intervene to try to stop the infighting.

Hamas moved systematically throughout the day, taking control of key Fatah positions. Fatah commanders complained they were not given clear orders by Abbas to fight back and that they had no central command. Fatah's strongman in Gaza, Mohammed Dahlan, has spent the last few weeks in Cairo for treatment of a knee injury.

At least 15 people were killed Wednesday, bringing the death toll in the four-day campaign to more than 50. Among those killed Wednesday was a man who joined a nonviolent protest of the fighting in Gaza City.

Hamas attacked the three main compounds of Fatah-allied forces in Gaza City ¡ª the headquarters of the Preventive Security, the Intelligence Service and the National Forces ¡ª in what could usher in the final phase of the battle.

The fighters, firing rockets and mortar shells, took over the rooftops in nearby houses and cut off the roads to prevent Fatah reinforcements from arriving.

Hamas gunmen in high-rise buildings also fired at Abbas' Gaza office and house and his guard force returned fire. Abbas was in the West Bank at the time.

Earlier, Hamas militants surrounded a security headquarters in the southern Gaza town of Khan Younis and ordered everyone inside to leave or they would blow the building up, witnesses said. The building was then destroyed by a bomb planted in a tunnel beneath it, said Ali Qaisi, a presidential guard spokesman.

Security forces later said they lost control of the town.

"Khan Younis is finished, but we are still holding on in Rafah," said Ziad Sarafandi, a senior security official, referring to a town south of Khan Younis. But soon after, Hamas militants blew up a second security building near Rafah after a long gunbattle, said Col. Nasser Khaldi, a senior police official.

"Hamas surrounded the building, they had come from Khan Younis to Rafah, they are working by plan," he said.

The Popular Resistance Committees, a militant group allied with Hamas, said it had taken control of Gaza's border with Egypt to prevent arms smuggling and to ensure that Gaze residents did not flee over the border.

Shops in Gaza City were closed, and streets were mostly empty as terrified residents huddled in homes. The U.N. Relief and Works Agency said it couldn't distribute food to the 30 percent of the Gaza Strip that relies on international aid.

The agency's Gaza director, John Ging, said the agency would curtail its operations after two of its Palestinian workers were killed by crossfire, but insisted, "We are scaling back, we are not pulling back."

Hamas and Fatah have waged a sporadic power struggle since Hamas won parliament elections last year, ending four decades of Fatah rule. But the battles have worsened as Hamas waged a systematic assault on security forces to take over Gaza.

Fighting between the two factions, which nominally share power in the Palestinian government, spilled into the Fatah-dominated West Bank. Militants exchanged fire in the city of Nablus and a nearby refugee camp, after Fatah gunmen tried to storm a pro-Hamas TV production company. Hamas said 12 people of its fighters were wounded.

Abbas appealed by phone to Hamas' exiled leader in Syria, Khaled Mashaal, to end the violence.

Hamas spokesman Sami Abu Zuhri, said the clashes could have been avoided if Abbas had given the Hamas-led Cabinet control over the security forces, which he blamed for a wave of kidnappings, torture and violence in Gaza.

"The president bears complete responsibility for the current crisis," he said.

The mounting bloodshed touched off protests in two main Gaza towns.

Several hundred tribal leaders, women, children and Islamic Jihad militants turned out in Gaza City for a protest initiated by Egyptian mediators. Some demonstrators scattered after masked Hamas gunmen fired in the air, but others pushed on, carrying Palestinian flags and shouting, "Do not shoot" and "national unity" over a loudspeaker.

Witnesses said Hamas gunmen shot at the protesters as they approached the home of Fatah loyalists, trapping them.

Protester Bilal Qurashali said he saw a man shot in the head. "We are unable to get out. The place is closed," he said.

Health officials said one protester was killed and 14 others were injured by bullets and brought to the hospital in civilian cars because ambulances couldn't navigate the heavy fire.

Separately, Hamas gunmen opened fire from a high-rise building at about 1,000 protesters in Khan Younis, injuring one and breaking up the protest.