There have been renewed clashes between protesters and police in the eastern Libyan city of Benghazi, witnesses say.
They say three people were killed in the city's el-Kish area and at least a dozen others elsewhere. The claims cannot be independently verified.
Benghazi has been the scene of protests in recent days, with reports that at least 15 people were killed in clashes with security forces on Thursday.
Reports are also coming about clashes in the neighbouring city of Al-Bayda.
Two exile groups told Reuters news agency that al-Bayda was now "out of the control of the Gaddafi regime".
It has been impossible for the BBC to verify this information on the ground.
Large public protests are rare in Libya, where dissent is seldom allowed by long-serving leader Col Muammar Gaddafi.
Pro-democracy protests have been sweeping through several Arab nations, with the presidents of Tunisia and Egypt forced from power amid growing unrest.
On Friday, thousands of people took to the streets of Benghazi for the second day, with many gathering outside the courthouse in the port city, witnesses say.
Some demonstrators were calling for change, while others said they only wanted more freedoms.
Three people were killed in the clashes with the police in el-Kish and a dozen others elsewhere, the witnesses say.
A medical source told CNN that 20 people were killed and 200 injured.
Several government and security offices were reportedly set ablaze during the demonstrations during the protests.
None of this information can be independently verified as foreign correspondents are banned from reporting from Benghazi.
A leading pro-government newspaper, Al-Zahf Al-Akhdar, called for tough action against the protesters.
"Any risk from these minuscule groups [protesters] - this people and the noble revolutionary power will violently and thunderously respond," the paper said.
"The people's power, the Jamahiriya [system of rule], the revolution, and Colonel Gaddafi are all red lines and those who try to cross or come near these lines are suicidal and playing with fire."
The US-based pressure group Human Rights Watch said at least 24 people had been killed across Libya in unrest on Wednesday and Thursday.
Many others were wounded in the clashes between security forces and protesters, the campaign group said.
Meanwhile the chief editor of the Quryna newspaper, Ramadan Briki, told the BBC that some prisoners had escaped from Benghazi's al-Kuifya prison and set fire to the local prosecutor's office, a bank and a police station.
Police later arrested 100. It was not clear if any prisoners remained at large, Mr Briki said.
A doctor at Benghazi's Jalla hospital told the BBC that he had seen 15 bodies - all dead from gunshot wounds - by the time he left the hospital in the early hours of Friday.
He said one of them was a 13-year-old boy.
Other witnesses claimed that six police cars in front of the hospital had been set on fire by relatives of the victims.
Our correspondent Jon Leyne in Cairo says violent confrontations are reported to have spread to five Libyan cities in demonstrations so far, but not yet to Tripoli, the capital, in any large numbers.
Activists supporting Col Gaddafi have also been out on the streets in Tripoli, chanting pro-government slogans in Green Square.
Col Gaddafi briefly visited the square in the early hours of Friday, according to images aired by state TV.
He is the Arab world's longest-serving leader, having ruled oil-rich Libya since a coup in 1969.