There are reports of protests by hundreds of people in the Libyan city of Benghazi.
Eyewitnesses told the BBC that the unrest had been triggered by the arrest of a lawyer who is an outspoken critic of the government.
The lawyer was later said to have been released but the protests continued.
Pro-democracy protests have swept through several Arab countries in recent weeks, forcing the leaders of Tunisia and Egypt from power.
A call has been put out on the internet for protests across Libya on Thursday.
'Police injured'
There is no independent confirmation of the overnight protests in Benghazi, but eyewitnesses say that at one stage some 2,000 people were involved.
They say stones were thrown at police who are said to have responded with water cannon, tear gas and rubber bullets.
Later, state television showed pictures of several hundred people in Benghazi voicing their support for the government. The government has so far not commented on events in the city.
Fourteen people were injured, including 10 police officers, Reuters quoted a report in the online edition of Libya's privately-owned Quryna as saying.
One witness, who did not want to be named, later told the BBC: "A couple of people in the crowd started chanting anti-government slogans and the crowd took that on.
"But then there were clashes with pro-government supporters and then after a bit the pro-government supporters were dispersed and then the security services arrived and they dispersed the crowds with hot-water cannons."
The protests began after the arrest of Fathi Terbil, who represents relatives of the alleged massacre of more than 1,000 prisoners by security forces in Tripoli's Abu Salim jail in 1996, reports say.
Sources say he was held after telling relatives of current inmates that the prison was on fire and urging them to protest. Mr Terbil was later said to have been freed.
Reports from Libya say that 110 members of a banned militant group will be freed from Abu Salim later on Wednesday. It is not clear if the Benghazi clashes and the planned release of the inmates are connected.
The reported unrest comes a day before planned anti-government demonstrations on Thursday, which are being organised via the internet.
Col Gadaffi, who has been in power since 1969, has suggested he might join the protests himself, the BBC's Jon Leyne reports.
He has always insisted that the country is run by a series of peoples' committees, though most outside observers believe it is a police state with Col Gadaffi firmly in control, our correspondent adds.
The Middle East has seen a wave of protests fuelled by discontent over unemployment, rising living costs, corruption and autocratic leaderships.
They 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.
