GAZA CITY, Gaza Strip (AP) – Hospitals ran out of ambulances, forcing people to evacuate the wounded in private cars, carrying bloodied victims in their arms into emergency rooms. One man rushed in with a boy who was shot in the head and later died of his wounds.

On the bloodiest single day of their violent power struggle, Fatah and Hamas gunmen battled in the streets of Gaza Friday, attacking two universities and a radio station and leaving 17 people dead, including four children.

Frightened residents huddled inside their homes, some calling local radio stations asking for help as armed men took position on their rooftops. Those who did venture outside took cover from the crossfire at the entrances to shuttered stores, or cowered behind walls. Thick smoke from explosions hovered over the Gaza sky line for hours.

As the death toll climbed, the two sides declared another truce. But they said they needed time to pull their volatile forces off the streets, and fierce gunbattles raged across Gaza for hours after the announcement.

A cease-fire declared earlier in the week collapsed Thursday, with gunmen armed with mortar shells, rockets and heavy guns trading fire across the Gaza Strip. In all, two dozen Palestinians were killed and some 250 were wounded in Thursday’s fighting.

In Washington, the so-called Quartet of Mideast negotiators met Friday to explore ways to jump-start peacemaking despite the violence among Palestinian factions.

“There’s simply no reason to avoid the subject of how we get to a Palestinian state,” Secretary of State Condoleezza Rice said after meeting with foreign ministers from the European Union, United Nations and Russia.

More than 100 Palestinians have been killed in internal violence since the Islamic militant Hamas, which rejects Israel’s right to exist, won parliamentary elections last year and ousted Palestinian President Mahmoud Abbas’ more moderate Fatah movement from power.

Friday’s fighting centered on two universities and a radio station – symbols of the factions’ prestige.

Fatah fighters stormed Islamic University in Gaza City, a Hamas stronghold, setting fire to two buildings and sparking a heavy firefight with Hamas forces. Masked men in black uniforms ran through the campus and took up positions on the roof of the school’s mosque.

Palestinian TV aired footage of dozens of rocket-propelled grenade launchers, rockets and assault rifles, as well as thousands of bullets that Abbas’ presidential guard said were confiscated at the university.

Hamas gunmen vowed revenge and hours later attacked two buildings of the Fatah-affiliated Al-Quds University in Gaza, Palestinian security officials said. A witness said gunmen fired mortar rounds at a building at the Gaza City campus, then doused it with fuel. Black smoke rose from the building as gunmen clashed outside.

Ahed al-Shawa, who lives opposite the Islamic University, cowered inside his apartment.

“We hear more than we see,” the 45-year-old said. “Even when you see shots flying, you don’t know who is firing at who.”

Al-Shawa said he hoped to venture outside for food and medicine, but wasn’t hopeful because firefights persisted outside his building after the truce announcement.

Most of Gaza City remained plunged in darkness after electrical facilities were damaged in gunbattles a day earlier. Hospital officials appealed for blood donations, saying they were running out of supplies to treat the wounded.

Also Friday, Hamas gunmen blew up the Fatah-affiliated Voice of Labor radio station in the northern town of Jebaliya after a five-hour siege, according to Rasem Bayri, who heads the Palestinian Federation of Labor Unions.

In Gaza City, 50 officers from Abbas’ presidential guard surrounded the Hamas-led Interior Ministry and exchanged fire with Hamas gunmen guarding the building. Five members of the presidential guard were killed.

Outside Gaza City, Hamas militants launched mortar rounds at a Fatah training base, wounding 30 recruits, security officials said. One shell missed the base, hitting a nearby house and wounding two children inside.

On Friday afternoon, leaders of Hamas and Fatah said they had agreed in principle to a new cease-fire, but needed to meet again to work out the details.

“We, the leaders of the two groups, agreed with God’s help on a cease-fire,” said Nizar Rayan, a regional Hamas leader, after the two sides met at the Egyptian Embassy in Gaza City. “The measures that will be taken on the ground will be discussed in the next few hours.”

A Fatah spokesman, Abdel Hakim Awad, confirmed agreement was reached in principle.

Later, the al-Jazeera TV station said Abbas and Hamas’ supreme leader, Khaled Mashaal, had agreed, in a phone call, to an immediate truce. The two are to meet Tuesday in Saudi Arabia for a new round of talks on forming a coalition government, said Nabil Abu Rdeneh, an Abbas spokesman.

Both Abu Rdeneh and a senior Damascus-based Palestinian official confirmed the two men spoke, but did not mention an immediate truce.

But the Damascus-based official said Mashaal phoned a meeting of Hamas and Fatah officials and emphasized the need for an immediate cease-fire.

The cease-fire talk did little to halt the fighting. Clashes were ongoing late Friday in at least seven spots across Gaza, including firefights near the bases of elite security forces loyal to Abbas.

Fatah spokesman Abdul al Hakim Awab said Hamas gunmen fired on a member of Fatah’s negotiating team as he drove to Abbas’ house, and one of his bodyguards was wounded. Egyptian mediators who were traveling in a convoy with the Fatah negotiator also came under fire, but escaped unharmed.

The U.N. said it would not reopen its schools in Gaza Saturday after a midyear recess because of the fighting. Nearly 200,000 students study at the schools.