Libyan troops have opened fire with machine-guns and large-calibre weapons on anti-government protesters in the second city Benghazi, witnesses say.
An unknown number of people, including children, are said to have been killed.
Witnesses described scenes of chaos as snipers shot from the roofs of buildings and demonstrators fought back against troops on the ground.
A doctor at a local hospital said he and his colleagues were treating hundreds of injured protesters.
Reports are difficult to verify as Libyan authorities have not allowed foreign journalists into the country, but residents say Benghazi and another eastern city, al-Bayda, appear to be out of government control.
There have been no reports of major protests in the capital Tripoli.
The Libyan leader, Colonel Muammar Gaddafi, is the Arab world's longest-serving leader, having ruled the oil-rich state since a coup in 1969.
Libya is one of several Arab countries to have experienced pro-democracy demonstrations since the fall of long-time Tunisian President Zine El Abidine Ben Ali in January. Egypt's Hosni Mubarak was forced from power on 11 February.
'Dozens killed' on Saturday
Benghazi, about 1,000 km (600 miles) from Tripoli, has been the main focus of the demonstrations against Col Gaddafi's 42-year rule.
Troops opened fire on people attending a funeral there on Saturday, killing 15, both the Associated Press news agency and al-Jazeera television said.
But an eyewitness told Reuters news agency that many more had actually died.
"Dozens were killed... not 15, dozens," the unnamed eyewitness said, adding that he had helped take victims to a local hospital.
The latest deaths take the toll over the past three days of protests in Libya to at least 100 dead. Some reports put it significantly higher.
A Benghazi resident told the BBC that security forces inside a government compound had fired on protesters with mortars and 14.5mm machine guns - a heavy machine gun typically produced in the former USSR.
They were, he said, machine-gunning cars and people indiscriminately. "A lot [of people] have fallen down today," he added.
Other witnesses spoke of snipers firing at protesters from rooftops and there were widespread reports of foreign mercenaries from sub-Saharan Africa being brought in to attack protesters.
A doctor told the BBC that situation in the city was "like hell", saying he had been seeing people with gunshot wounds being carried into his hospital all day.
In an appeal sent to Reuters news agency, a group of religious and clan leaders from across Libya urged "every Muslim, within the regime" or anyone helping it: "Do NOT kill your brothers and sisters, STOP the massacre NOW!"
Another Benghazi resident told Reuters that the government compound was the only part of the town still under military control.
"The rest of the city is liberated," he said.
He added that local government offices and police stations in the city had been burnt down and thousands of protesters were organising around the courthouse, equipped with makeshift clinics, ambulances, speakers and electricity.
His account could not be verified independently, and Libyan state media have ignored the unrest, focusing instead on pro-Gaddafi rallies in Tripoli.
Human Rights Watch put the death toll since Wednesday, when protests erupted, at 104, not counting those killed overnight on Saturday. The New York-based group stresses that it is a conservative estimate.
UK Foreign Secretary William Hague said reports of heavy weapons and sniper fire being used on protesters were unacceptable.
"This is clearly unacceptable and horrifying," Mr Hague said, adding: "The absence of TV cameras does not mean the attention of the world should not be focused on the actions of the Libyan government."
The British Foreign Office is now advising UK citizens against all but essential travel to Benghazi, Ajdabiya, al-Bayda, al-Marj, Darnah, Ajdabiya, Tobruk and areas bordering Sudan, Chad, Niger and Algeria.