Witnesses say snipers using live rounds on crowds, Muslim leaders urge "Stop the massacre now!"
Tens of thousands gathered in Benghazi on Sunday for funerals of protesters killed by Libyan security forces as Human Rights Watch said overnight violence had doubled the death toll from four days of clashes to 173.
The unrest, the worst in Libyan leader Muammar Gaddafi's four decades in power, started as a series of protests inspired by popular revolts in neighbouring Egypt and Tunisia, but was met by a fierce response.
Reporters have not been allowed into the city but piecemeal witness accounts suggest it is in a cycle of violence, where people are killed and then, after funeral processions to bury the dead the next day, security forces shoot more protesters.
"A massacre took place here last night," one Benghazi resident, who did not want to be named, said on Sunday.
He said security forces had used heavy weapons, adding: "Many soldiers and policemen have joined the protesters."
Another resident, who also declined to be named, said: "Some 100,000 protesters are now heading for a cemetery to bury dozens of martyrs."
A Benghazi hospital doctor said victims had suffered severe wounds from high-velocity rifles.
Another witness, a leading tribal figure who requested anonymity, suggested the security forces remained confined to their control centre.
"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 said. "People are running their own affairs."
The New York-based Human Rights Watch said about 90 people had been killed on Saturday in clashes centred on Benghazi and surrounding towns running into the night, taking the death toll from four days of clashes to 173.
Conflicting accounts were given over poor phone lines but it appeared the streets were under the control of protesters while security forces had pulled back to a high-walled compound, known as the Command Centre, from where they were firing.
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 said 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.
The Libyan government has not released any casualty figures or made any official comment on the violence.
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 capital Tripoli, 1,000km (600 miles) to the west, and the rest of the country.
The crackdown prompted about 50 Libyan Muslim religious leaders to issue an appeal 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 recognise 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, 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."
Some analysts have said there may be negotiations between Gaddafi and eastern tribal leaders, and an SMS message sent late on Saturday to Libyan mobile phone subscribers hinted at a more conciliatory approach.
"All citizens and youth of Benghazi, those who died among the civilians and police are all sons of our country. Enough of what has happened and stop the bloodshed."
In Tripoli, several thousand Gaddafi supporters rallied in the city's Green Square until the early hours of Sunday morning.
They chanted "God, Libya and Muammar!" and "Muammar is the pioneer of Arab nationalism!".
Libya's state news agency said some cities had seen acts of arson and vandalism, and blamed "a foreign network trained in creating clashes and chaos so as to destabilise Libya".
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.
Witness accounts have been hard to verify independently because Libyan authorities have not allowed foreign journalists into the country since the protests erupted and local reporters have been barred from travelling to Benghazi.
Mobile phone connections have often been out of service and Internet service in Libya has been cut off, according to a US company that monitors web traffic. People in Tripoli said they had Internet access late on Saturday.
Many of these regime leaders feel that they will not give up unless they die or die trying to defend their regime. Saddam had that same analogy but the US chased him down and hung him. I feel sorry for all those dying in Libya and for those that died in Bahrain as well. It is obvious that the government issues these orders to kill a few to put fear into the protesters mind to test their limits. In the end no one knows how far each side will go before one side backs down. One thing is clear and that is people want freedom in the middle-east. Monopolies and censorship need to come to an end.
ArabianBusiness is censoring too many comments about how the people feel about these protests and riots. No wonder more riots and protests are taking place in the middle-east. Silencing the people is not right and will not help the middle-east. DFM tanked badly yesterday. I see a Domino affect taking place.
Andy is correct. There is to much censoring by AB. Why be afraid of the truth. Why even work where the truth does not prevail. Arabian Business should be supporting those seeking freedom from brutal monarchs.
The Libyan revolution, with its tribal traditions is the most poignant for the Gulf region so far. The desire for justice and freedom of the twitter generation in Libya has challenged and overcome the worlds most brutal and cynical petro dollar financed regime. Every Arab tribal society is paying close attention.