Speaking at a UN human rights conference in Geneva, US Secretary of State Hillary Clinton said Libyan leader Col Gaddafi must "go now".
The EU on Monday imposed sanctions including an arms embargo, asset freeze and travel ban on Libyan authorities, including Col Gaddafi.
Tens of thousands of migrants - many from Egypt - are stranded near Libya's Tunisian border, UN officials say.
Unrest continues in and around Tripoli, with a demonstration against Col Gaddafi in the capital's suburb of Tajoura, with protesters chanting: "The blood of martyrs won't go to waste."
And reports from the coastal town of Misrata say Col Gaddafi's opponents have repelled a government counter-attack.
Witnesses said fighting had been taking place around the town, 200km east of Tripoli, and its airport.
'Braving the bullets'
In Geneva, Mrs Clinton accused Col Gaddafi and his followers of using "mercenaries and thugs" to attack unarmed civilians, and of executing soldiers who refused to turn their guns on fellow citizens.
"Gaddafi and those around him must be held accountable for these acts, which violate international legal obligations and common decency," she said.
"It is time for Gaddafi to go, now, without further violence or delay."
Mrs Clinton said the Libyan protesters were "braving the dictator's bullets and putting their lives on the line to enjoy the freedoms that are the birthright of every man, woman and child on Earth".
Opening the meeting, UN Human Rights Commissioner Navi Pillay warned the Libyan authorities that widespread attacks on civilians could amount to crimes under international law.
"The illegal and excessively heavy-handed response of a number of governments is unacceptable," she said.
European Union foreign policy chief Catherine Ashton said the organisation's sanctions on Libya would take effect quickly. They reinforce Saturday's UN Security Council measures and target Col Gaddafi and 25 members of his close entourage.
"What is going on - the massive violence against peaceful demonstrators - shocks our conscience," she said.
British Foreign Secretary William Hague said Libya had "failed shamefully in its responsibilities to its people".
Russian Foreign Minister Lavrov said the use of military force against the civilian population was "unacceptable".
His Australian counterpart Kevin Rudd called for a no-fly zone to protect the Libyan people, and said Col Gaddafi should depart.
"The peoples of the entire world are saying 'go'. And for the sake of humanity, 'go now'," he said.
In other developments:
* German Foreign Minister Guido Westerwelle says he is proposing a 60-day freeze on all financial payments to Libya to prevent Col Gaddafi from "oppressing his people"
* At a news conference, Libyan government spokesman Moussa Ibrahim denied any massacres or bombardments against cities, saying UN resolution 1970 had been formulated based on media reports
* Opposition forces controlling eastern cities - including Benghazi, where the uprising started 10 days ago - say they have formed a national council to act as the political face of the anti-Gaddafi movement
* Anti-government forces now control the town of Zawiya, just 50km (30 miles) from Tripoli but pro-Gaddafi forces are surrounding the city
* On Sunday Italy's foreign minister said Italy had de facto suspended its 2008 friendship treaty with Libya, which includes a non-aggression clause
* A Ukrainian nurse described as a "confidante" of Col Gaddafi, returns to Kiev with other evacuees
* France says it is sending two planes carrying medical aid, in what Prime Minister Francois Fillon described as the start of a "massive humanitarian aid operation"
Transit camp
About 100,000 people have fled anti-government unrest in Libya over the past week, the UN estimates.
The exodus of Egyptian workers from western Libya began on Wednesday, but has since been intensifying, says the BBC's Jim Muir at the Ras Jdir border crossing with Tunisia. About 1,000 people an hour are crossing into Tunisia, he says.
Although more aircraft and ships are due in Tunisia to accelerate the evacuation of migrant workers, the country's authorities are no longer able to cope with the influx, the UN refugee agency's Liz Eyster told the BBC.
"They've been accommodating people in shelters, schools and places of their own," she said. "But we're now aware of the fact that they're very much stretched and they need the support of the international community."
Col Gaddafi is facing the biggest challenge to his 41-year rule, but still controls the capital Tripoli.
Meanwhile the BBC's Kevin Connolly in Benghazi says eastern Libya is getting back to some muted form of normality, but it is not clear what will happen when the money dries up.
Also, there is no rebel army to make the 1,600km (1,000-mile) journey across the desert to storm Col Gaddafi's last stronghold, he says.
Late on Saturday the UN Security Council unanimously backed an arms embargo and asset freeze on senior Libyan government officials.
It also voted to refer Col Gaddafi to the International Criminal Court for alleged crimes against humanity.
source : bbc.co.uk
