At least 24 people have been killed in anti-government protests in Libya in recent days, rights activists say.
Many others were wounded in the clashes between security forces and protesters, the US-based Human Rights Watch said.
Protests continued overnight with thousands on the streets of the eastern city of Benghazi, where there is now a heavy military presence, witnesses said.
Large protests are uncommon in Libya, where dissent is rarely allowed.
Pro-democracy protests have recently swept through several Arab nations, with the presidents of Tunisia and Egypt forced from power amid growing unrest.
The BBC's 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.
Our correspondent says the reports reflect an extremely tough government response, including the use of gunfire and even denying supplies to hospitals.
Funerals of some of those killed are expected to be held on Friday in Benghazi and al-Bayda, which correspondents say could spur more protests.
Activists set up camps in al-Bayda after Thursday's "Day of Rage" protest against the government, witnesses said.
Eyewitnesses believe that the death toll could be even higher, our correspondent says.
Activists supporting Libya's leader, Muammar 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, AFP news agency reports.
In Benghazi, an eyewitness said that most protesters went home by 0500 local time (0700 GMT) after running battles with police in several districts overnight.
A source in the city told the BBC that "several" peaceful protesters who had stayed put overnight in front of the courthouse were forcibly removed in the very early hours on Friday by a large number of police.
They have now reportedly been taken to a detainment facility in Gwarsha, about 10km (6 miles) outside the city.
Correspondents say the city was calm on Friday morning, but there are an enormous number of security personnel on the streets.
It seems to be in preparation for what they fear will be an even bigger demonstration today after Friday prayers, correspondents say.
A resident who lives on Benghazi's main thoroughfare, Nasser Street, told Reuters on Friday morning the city was much quieter than the night before.
"Last night was very hard, there were a lot of people in the street, thousands of people. I saw soldiers in the street," he said.
"I heard shooting. I saw one person fall down (from a gunshot wound)."
Activists had used social networking websites such as Facebook and Twitter to call for Thursday's "Day of Rage".
Human Rights Watch says hundreds of peaceful demonstrators had taken to the streets in al-Bayda, Benghazi, Zintan, Darna, and Ajdabiya.
It quotes witnesses as saying the Libyan security forces shot and killed demonstrators in efforts to disperse the crowds.
A doctor from Jalla hospital in Benghazi told the BBC that by 0300 on Friday, 15 dead bodies had arrived, including a 13-year-old boy.
All had been killed by gunshot wounds, the doctor said.
Six police cars in front of Jalla hospital were set alight by angry parents and relatives of victims and the injured, according to an eyewitness on Friday.
One protester, speaking to the BBC late on Thursday, said he had seen three demonstrators killed in al-Bayda.
"The police are using their guns... I have a video which shows the police shooting people but the government has blocked the internet in al-Bayda. We are asking the authorities to unblock the internet service.
"The biggest problem now is that doctors are not treating the injured and so they die. It's a crime," he said.
Col Gaddafi is the Arab world's longest-serving leader, having ruled oil-rich Libya since a coup in 1969.
The Middle East has recently seen a wave of protests fuelled by discontent over unemployment, rising living costs, corruption and autocratic leaderships.
This began with the overthrow of Tunisia's leader, Zine al-Abidine Ben Ali, in January. Protests in Egypt then lead to the resignation of President Hosni Mubarak.
In recent days there have also been anti-government demonstrations in Yemen, Bahrain, and Iran.