Members of a Libyan army unit told Benghazi residents on Sunday they had defected and "liberated" the city from forces supporting veteran leader Muammar Gaddafi, two residents said.
Habib al-Obaidi, who heads the intensive care unit at the main al-Jalae hospital, and lawyer Mohamed Al-Mana, told Reuters members of the "Thunderbolt" squad had arrived at the hospital with soldiers wounded in clashes with Gaddafi's personal guard.
"They are now saying that they have overpowered the Praetorian Guard and that they have joined the people's revolt," Mohamed said by telephone. It was not possible to independently verify the report.
Meanwhile, Libya's permanent representative to the Arab League, Abdel Moneim al-Honi, said on Sunday he was quitting his position in order to "join the revolution" that is unfolding in his country.
Obaidi said the bodies of 50 people killed on Sunday had arrived at the hospital in the late afternoon. Most had died from bullet wounds.
Sunday's bloodshed follows the deaths of scores of protesters on Saturday in one of the most violent days since protests began sweeping through the Arab world two months ago.
Residents said tens, perhaps hundreds of thousands of protesters had taken to the streets of the city to bury scores of dead killed in the last 24 hours. A witness said security forces opened fire on them.
In Egypt, exiled Libyans and members of the country's Press Syndicate have sent urgent medical supplies to Libya. Ayman Shawki, a lawyer in the Egyptian border town of Matrouh, said members of the powerful Awllad Ali tribe whose members live in the border area have volunteered to move the supplies to Libya.
The United States said it was "gravely concerned" by what it called credible reports hundreds of people had been injured or killed.
"Libyan officials have stated their commitment to protecting and safeguarding the right of peaceful protest. We call upon the Libyan government to uphold that commitment and hold accountable any security officer who does not act in accordance with that commitment," said State Department spokesman Philip Crowley.
Hundreds of fearful Tunisians fled what they called "real carnage" in Libya on Sunday to head home via the coastal Ras Jdir border crossing, a union official told AFP.
The Tunisians said they fled for their own safety after what they described as "real carnage under way in Libya", said Houcine Betaieb, a member of Tunisia's influential UGTT trade union.
"Hundreds of Tunisians left Libya Sunday through the Ras Jdir border post. There are a lot of people and there is a big bottleneck in the area," he said.
He said five buses had been made available free-of-charge to the fleeing Tunisians, many of whom left so quickly there were carrying no money.
"These are people who work there, who have left Libya out of fear that something would happen to them," he said.
According to reports from the eastern city of Benghazi the military used helicopters to drop security forces, including mercenaries from Sub-Saharan Africa, in cities overtaken by protesters.
Witnesses said there was arbitrary shooting on Benghazi residents , including women and children.
In a telephone call, an eyewitness from Benghazi called for an immediate international intervention for what he said was a "genocide being committed by Gaddafi."
Reporters have not been allowed into Libya's second city but piecemeal accounts suggest its streets are largely under the control of anti-government protesters, who periodically come under attack from security forces firing out of their high-walled compound.
"A massacre took place here last night," one Benghazi resident, who did not want to be named, told Reuters by telephone on Sunday.
He said security forces had used heavy weapons, adding: "Many soldiers and policemen have joined the protesters."
Another resident of Benghazi, about 1,000 km (600 miles) east of the capital Tripoli, told Reuters: "Some 100,000 protesters are now heading for a cemetery to bury dozens of martyrs."
If the procession followed the pattern of the previous few days, it was likely to be fired on again from the Command Centre, starting a fresh round of fighting and mourning.
A Benghazi hospital doctor said overnight victims had suffered severe wounds from high-velocity rifles.
Another witness, a leading tribal figure who requested anonymity, said security forces were confined to their compound.
"The state's official presence is absent in the city and the security forces are in their barracks and the city is in a state of civil mutiny," he told Reuters. "People are running their own affairs."
The witness who spoke of the funeral procession gathering said: "We fear a new massacre because the road leading to the cemetery is not far from a security barracks.
"We will not give up until the regime falls. We call on the United Nations to intervene immediately to stop the massacre."
Another witness in Benghazi told Reuters thousands of people had performed ritual prayers in front of 60 bodies laid out near Benghazi's northern court.
He said hundreds of thousands of people, including women and children, had come out onto the Mediterranean seafront and the area surrounding the port. "The protesters are here until the regime falls," he said.
Libyan analysts say it is unlikely for the moment that Gaddafi will be overthrown because the unrest is largely confined to the eastern Cyrenaica region, where his support has traditionally been weaker than in the rest of the country.
The Libyan government has not released any casualty figures. A text message sent to mobile phone subscribers on Sunday said the protesters in the east were trying to break the region away from central rule.
"The deaths in Benghazi and Al Bayda (a nearby town), on both sides, were the result of attacks on weapons stores to use in terrorizing people and killing innocents," it said.
"All Libyan sons, we have to all stand up to stop the cycle of separation and sedition and destruction of our beloved Libya."
The crackdown prompted about 50 Libyan Muslim religious leaders to issue an appeal, sent to Reuters, for the security forces, as Muslims, to stop the killing.
"We appeal to every Muslim, within the regime or assisting it in any way, to recognize that the killing of innocent human beings is forbidden by our Creator and by His beloved Prophet of Compassion (peace be upon him) ... Do NOT kill your brothers and sisters. STOP the massacre NOW!" the appeal said.
Foreign reaction to the unrest in Libya, a major energy producer with significant investment from Britain's BP Plc, Exxon of the United States and Italy's ENI among others, has so far been muted.
But Britain called on Sunday for a stronger response.
"The world should not hesitate to condemn those actions," Foreign Secretary William Hague told Sky News.
"What Colonel Gaddafi should be doing is respecting basic human rights, and there is no sign of that in the dreadful response, the horrifying response, of the Libyan authorities to these protests."
Libya was for decades under U.S. and European sanctions over its banned weapons programs, so Gaddafi is unlikely to be alarmed at the prospect of new international isolation.
Sir Richard Dalton, a former British ambassador to Libya, told the Independent on Sunday newspaper that Gaddafi would find it hard to make concessions in order to survive.
"I think the attitude of the Libyan regime is that it's all or nothing," he said.