The latest ceasefire was put under pressure almost as soon as it began.
A negotiating team from Fatah came under fire as it left the Egyptian-mediated talks in Gaza City, and two bodyguards were injured.
Running gun battles
Residents of the Gaza Strip hid in their homes on Friday as armed men fought running gun battles firing rifles and rocket-propelled grenades from streets and rooftops.
"It is pitiful to witness the bloody scenes in Gaza between the brothers"
"Gaza is being burned down," Arafat Abu Eyad told Reuters news agency from his balcony overlooking smouldering buildings.
Hamas fighters overran several compounds used by Abbas's Fatah movement, while Fatah members also stormed and burnt Gaza's Islamic university.
Three members of the Palestinian intelligence service, including a deputy chief commander, were killed in fighting with Hamas fighters in the Jabalya refugee camp, security and medical sources said.
Dozens of members of Abbas' presidential guard were wounded when Hamas fighters and members of the controversial Hamas "executive force" controlled fired a mortar into their training camp in Gaza City.
"Between 40 and 50 new recruits of the presidential guard were wounded and there are perhaps some killed," one Fatah official said.
University set on fire
Presidential guardsmen stormed the Islamic University in Gaza City overnight, a known Hamas stronghold.
An explosion took place inside one of the campus buildings causing a large fire, witnesses said, at what is reputedly the most prestigious Islamic university in the impoverished Palestinian territory.
|The Islamic University in Gaza was |
stormed by Fatah fighters [AFP]
Ismail Radwan, a Hamas spokesman, said: "We heavily condemn this action of the presidential guard and its acts of vandalism committed at the Islamic University, which is an academic institution without armed men or stashes of weapons."
Several armed men were detained, while weapons and explosives were confiscated during an initial sweep of the campus, a security source said.
Fatah said the operation had been ordered after Hamas fighters fired mortar shells and rocket-propelled grenades towards Gaza's main presidential compound from the university.
Hamas supporters responded by attacked two buildings of the Fatah-affiliated Al-Quds University at two separate locations in Gaza, Palestinian security officials said. A witness said mortar rounds were fired at the Gaza City campus.
Hamas fighters also blew up the Fatah-affiliated Voice of Labour radio station in the northern town of Jabalya after a five-hour siege, according to Rasem Bayri, who heads the Palestinian Federation of Labour Unions.
A ceasefire hammered out earlier in the week crumbled on Thursday when six Palestinians were killed and another 70 wounded, after Hamas ambushed a presidential guard supply convoy from Egypt.
One guard was left dead although Fatah denied the trucks were transporting weapons.
|Hamas attack on a Fatah supplies convoy on|
Thursday scuttled a ceasefire deal [AFP]
Weapons are being supplied by key US allies Jordan and Egypt, with US encouragement and Israeli approval.
Hamas has struggled to govern since taking office in March under the weight of US-backed sanctions imposed over its refusal to recognise Israel, renounce violence and accept existing interim peace deals with Israel.
Diplomats say Abbas's military build-up was meant to counter moves by Hamas in smuggling more powerful weapons into Gaza.
Israel and other Western countries have accused Iran, under world scrutiny for its nuclear programme, of supporting Hamas financially.