Libyans are bracing for mass protests as the rebel movement called for a new push to oust Colonel Muammar Gaddafi after a day of fierce fighting.
At least 17 people died in clashes as rebels made new gains and advanced closer to his stronghold in Tripoli and pro-government forces attacked two nearby cities.
International momentum has been building for action to punish Col Gaddafi's regime for the bloodshed.
The European Union's foreign policy chief, Catherine Ashton, said the EU should consider sanctions such as travel restrictions and an asset freeze against Libya to achieve a halt to the violence there and move toward democracy.
Nato's main decision-making body also planned to meet in emergency session on Friday to consider the deteriorating situation, although Secretary-General Anders Fogh Rasmussen has said the alliance has no intention of intervening in the North African nation.
The UN's top human rights official, Navi Pillay, meanwhile, said there are reports of mass killings of thousands in Libya that should spur the international community to "step in vigorously" to end the crackdown against anti-government protesters.
Most of the eastern half of Libya has already broken away, and diplomats, ministers and even a high-ranking cousin have abandoned Gaddafi, who has ruled Libya for 41 years.
He is still believed to be firmly in control only of the capital, some towns around it, the far desert south and parts of Libya's sparsely populated centre. Residents in several cities said the opposition had called on people to rally in demonstrations after Friday prayers.
Meanwhile, Col Gaddafi's son pledged that his family would "live and die in Libya".
Saif al-Islam Gaddafi told Turkish journalist Cuneyt Ozdemir: "Plan A is to live and die in Libya, Plan B is to live and die in Libya, Plan C is to live and die in Libya."