Members of a Libyan army unit have told Benghazi residents they had defected and "liberated" the city from veteran leader Colonel Muammar Gaddafi.
Habib al-Obaidi, who heads the intensive care unit at the main Al-Jalae hospital, and lawyer Mohamed al-Mana said members of the Thunderbolt squad had arrived at the hospital in Libya's second city 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," Mr al-Mana said by telephone.
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.
The United States said it was "gravely concerned" by what it called credible reports hundreds of people had been injured or killed.
The Foreign Secretary, William Hague, urged Libya to begin dialogue with anti-government protesters and implement reforms, in a phone call to a son of Gaddafi on Sunday.
In Brussels, the Hungarian EU presidency said Libya had told the European Union it would stop cooperation with the bloc in stemming illegal migration to Europe if the EU encourages pro-democracy protests in the country.