The opposition groups have asked people in Gaddafi stronghold Tripoli to join the rebellion. Supporters of the revolt in Tripoli have said they are planning to protest in Green Square after Friday prayers.
The Libyan protest movement has caused Gaddafi's government to lose control of much of the eastern part of the country.
In an address on state television Thursday, Gaddafi blamed Al-Qaeda's Osama bin Laden for the protests and claimed that hallucinogenic drugs had been given to young people to incite them to revolt.
Fighting was reported in towns near Tripoli, as pro-Gaddafi forces tried to regain control of areas seized by the opposition.
Reports citing witnesses in Zawiya, 50 km to the west, said an military unit had attacked protesters with machine guns and rocket-propelled grenades.
US President Barack Obama has spoken by telephone with the leaders of Italy, Britain, France and Italy as international efforts to respond to the crisis in Libya gather pace.
Obama outlined a range of possible measures, including plans for humanitarian assistance.