US President Barack Obama has denounced the violent crackdown by the Libyan authorities on peaceful protesters as "outrageous and unacceptable".
Mr Obama said the world had to speak with "one voice", and that the US was drawing up a range of options for action in consultation with its allies.
The Libyan government would be held accountable for its actions, he added.
His comments came as Muammar Gaddafi battled to keep control of western Libya, including the capital, Tripoli.
Opposition protesters - supported by many defecting government troops - have consolidated their control of the east of the country.
Residents of Tripoli have said they are too frightened to venture out, because of fears that pro-government forces will shoot them on sight.
Thousands of foreigners are meanwhile still trying to flee Libya through ports, airports, and the Tunisian and Egyptian borders. The UK Foreign Office is advising against all but essential travel to Libya, while efforts to evacuate Britons from Libya are being stepped up.
The overall death toll has been impossible to determine. Human Rights Watch says it has confirmed nearly 300 deaths, but the International Federation for Human Rights says at least 700 people have been killed.
'Full range of options'
In his first public comments on the unrest in Libya, Mr Obama did not criticise Col Gaddafi directly, but did condemn strongly the use of violence by his supporters to suppress those demanding he step down.
"The suffering and bloodshed is outrageous and unacceptable," he said from the White House. "So are threats and orders to shoot peaceful protesters and further punish the people of Libya."
Mr Obama said he had ordered his administration team to prepare the "full range of options" for dealing with the crisis, including unspecified actions that the United States could take alone or with its allies.
US Secretary of State Hillary Clinton is set to travel to Geneva on Monday, where foreign ministers will convene a session of the UN Human Rights Council.
In Brussels, EU ambassadors said the bloc was ready to impose further measures against Libya if necessary.
The BBC's Kim Ghattas in Washington says Mr Obama sounded determined and outraged, but offered little in terms of concrete action to help end the violence - other than sending Mrs Clinton to Europe.
And Mr Obama's request for the international community to speak with one voice suggests there are divisions about how to deal with Col Gaddafi, she adds.
Earlier, jubilant demonstrators took to the streets of the eastern cities of Benghazi and Tobruk waving flags, honking horns and setting off fireworks in what correspondents described as a giant party.
"We have been suffering for 41 years," Hamida Muftah, a resident of Benghazi, told Reuters. "Gaddafi has killed people... We are a very rich country, but most of the people are poorer than poor."
A number of military units in the east now say they have unified their command in support of the protesters, while a growing number of towns have set up informal opposition governments to fill the power vacuum.
The government has not yet attempted to regain control of the east, except around the town Ajdabiya, where security forces and militia are reportedly clashing with protesters along the road to Col Gaddafi's hometown of Surt.
Witnesses said Tripoli was largely deserted, with many fearing pro-government forces would shoot them if they ventured out.
"Anti-government protesters have disappeared. The streets are quiet. There are many, many deaths," one resident told BBC Arabic.
Two naval gunships are reported to have been deployed along the coast.
Col Gaddafi has urged his supporters to attack the "cockroaches" protesting against his rule, and "cleanse Libya house by house".
Despite the threats, opposition supporters said they were making plans for their first co-ordinated demonstration in the capital on Friday.
Reports from Misurata - Libya's third city, 210km (130 miles) from the capital - say security forces opened fire on demonstrators, killing several.
Clashes were also reported in the western towns of Zawiya and Sabratha, where witnesses said troops and foreign mercenaries had been deployed after demonstrators burned government buildings.
Despite the violence, Col Gaddafi's son, Saif al-Islam, went on television on Wednesday evening to say that everything was "normal".
"The ports, schools and airports are all open," he said. "The problem lies in the eastern regions. Life is normal. Brothers, Libyans should come together in this national battle."
State media also claimed that the country's long-serving Interior Minister, Gen Abdel Fattah Younes al-Abidi, had been kidnapped by "gangs" after he said he had resigned because the people of Benghazi had been shot at.
Rebels braced for counter-attack
Jon Leyne reports from the rebel-held city of Benghazi, where people have been celebrating an end to Colonel Gaddafi's rule in the east of the country. But they're beginning to realise the fight for control of the whole country is a long way from over.
Rebels hold town in Sahara
Ian Pannell has been to an area of Libya's Sahara desert, now in the hands of rebels. He finds the threat that Colonel Muammar Gaddafi's forces could return ever present and the rebels' hold on their new territory precarious.
Libya's front line
John Simpson in Aqayla in central Libya, which he says it is "not like the normal front line of a war zone".
Rescue mission starts
Ben Brown on the Libyan-Tunisian border, where an international effort has begun to rescue thousands of stranded people.
Navigate the map to see the latest reports from correspondents across the region as the crisis unfolds.
Protests in the capital had centred on Green Square and various key buildings, like the headquarters of state TV and the People's Hall, were attacked and damaged. But Libyan leader, Muammar Gaddafi, and his supporters are very much in control of Tripoli. Colonel Gaddafi has appeared several times on television from his compound in Bab al-Azizia making defiant speeches condemning the protests.
The Libyan Army is a weak force of little more than 40,000, poorly armed and poorly trained. Keeping the army weak is part of Colonel Muammar Gaddafi's long-term strategy to eliminate the risk of a military coup, which is how he himself came to power in 1969. The defection of some elements of the army to the protesters in Benghazi is unlikely to trouble the colonel. His security chiefs have not hesitated to call in air strikes on their barracks in the rebellious east of the country.
Libya produces 2.1% of the world's oil. Since the protests began, production has dropped, although Saudi Arabia has promised to make up any shortfall. The high revenue it receives from oil means Libyans have one of the highest GDPs per capita in Africa. Sirte basin is responsible for most of Libya's oil output. It contains about 80% of the country's proven oil reserves, which amount to 44 billion barrels, the largest in Africa.
Most of Libya's 6.5m poplation is concentrated along the coast and around the country's oilfields. Population density is about 50 persons per square kilometre along the coast. Inland, where much of the country is covered by inhospitable desert, the population density falls to less than one person per square kilometre.
A French doctor working in Benghazi, Gerard Buffet, told the BBC that he had operated day and night for four days on people injured by bullets, mortars and rockets.
"On the third day the planes above the city shot on the people. The police made a wall of dead people around the police building," he said.
Meanwhile, a massive evacuation operation is in full swing as other nations try to rescue thousands of their citizens caught up in the chaos.
The US, China and many European countries have sent in planes, ships and ferries to help people flee. Thousands of people, many of them African migrants, have also poured across Libya's land borders, in vans piled high with furniture and luggage.
The International Organisation for Migration says several governments have asked it to help evacuate their citizens.
The price of oil has jumped sharply in reaction to the continuing crisis in Libya. In New York, it passed $100 (£62) a barrel for the first time since 2008.