There were reports of clashes in two cities late on Wednesday, with about four people reported dead in the eastern city of al-Bayda.
Dozens of people were injured in violent demonstrations on Tuesday night in the eastern city of Benghazi.
The unrest there followed the detention of an outspoken government critic.
Pro-democracy protests have recently swept through several Arab nations, with the presidents of Tunisia and Egypt forced to resign amid growing unrest.
But this week’s demonstrations were the first display of defiance in Libya, where dissent is rarely tolerated.
Petrol bombs
A newspaper connected to one of Libyan leader Col Muammar Gaddafi’s sons, Libya al-Yawm, showed the police station in al-Bayda on fire.
There are a number of unconfirmed reports of the government opening fire on the protesters.
Reports on social networks talk of government forces possibly firing from helicopters on to the crowds. Other sources said snipers were used.
Exiled groups say at least four people were killed but figures are impossible to verify.
Witnesses say that at one stage up to 2,000 people were involved in the protests early on Wednesday in Benghazi, which saw a march on government offices in the city.
The protesters are said to have thrown stones and petrol bombs and set vehicles alight. Witnesses said police used rubber bullets and water cannon to disperse them.
The protests reportedly began after the arrest of Fathi Terbil, who represents relatives of more than 1,000 prisoners allegedly massacred by security forces in Tripoli’s Abu Salim jail in 1996. He was later said to have been freed.
In a speech broadcast on Wednesday evening, Col Gaddafi made no mention of the unrest but said the “revolutionaries” would prevail.
“Down with the enemies, down with them everywhere; down with the puppets everywhere, the puppets are falling, the autumn leaves are falling!” he said.
“The puppets of the USA, the puppets of Zionism are falling.”
In a statement issued after the Benghazi clashes, a senior Libyan official warned that the authorities “will not allow a group of people to move around at night and play with the security of Libya”.
It added: “The clashes last night were between small groups of people – up to 150. Some outsiders infiltrated that group. They were trying to corrupt the local legal process which has long been in place.
“We will not permit that at all, and we call on Libyans to voice their issues through existing channels, even if it is to call for the downfall of the government,” said the official, who was not identified.
More than 100 members of a banned Islamist militant group were freed from Abu Salim on Wednesday. It is not clear if the Benghazi clashes and the release of the inmates were connected.
The European Union, meanwhile, has urged Libya to allow “free expression”.
“We also call for calm and for all violence to be avoided,” said a spokeswoman for EU foreign policy chief Baroness Ashton.
‘Police state’
Col Gaddafi is the Arab world’s longest-serving leader, having ruled oil-rich Libya since a coup in 1969.
He has always insisted that the country is run by a series of people’s committees, though most outside observers believe it is a police state with him firmly in control, the BBC’s Jon Leyne reports.
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. Last week, President Hosni Mubarak of Egypt resigned.
In recent days there have also been anti-government demonstrations in Yemen, Bahrain, and Iran.