Details have emerged of huge casualty figures in the Libyan city of Benghazi, where troops have launched a brutal crackdown on protesters.
More than 200 people are known to have died, doctors say, with 900 injured.
The most bloody attacks were reported over the weekend, as funeral marches were said to have come under machine-gun and heavy weapons fire.
One doctor, speaking amid the sound of fresh gunfire on Sunday, told the BBC that "a real massacre" had happened.
Human Rights Watch says at least 173 people have been killed in Libya since demonstrations began on Wednesday.
Benghazi, the country's second city, has been a leading focus of protests against four decades of rule by Col Muammar Gaddafi.
Western politicians condemned the violence in Libya. During a phone conversation with Col Gaddafi's son, Saif al-Islam, UK Foreign Secretary William Hague spoke of London's "grave concern" at the escalation of violence, a statement said.
Libya's actions were "unacceptable and would result in worldwide condemnation", Mr Hague said.
The US state department said it was "gravely concerned with disturbing reports and images coming out of Libya".
"We are working to ascertain the facts, but we have received multiple credible reports that hundreds of people have been killed and injured in several days of unrest - and the full extent of the death toll is unknown due to the lack of access of international media and human rights organisations," spokesperson Philip Crowley said in a statement.
Mr Crowley said the US had raised their concerns with Libyan officials, including Foreign Minister Musa Kusa.
France was "extremely worried" by events in Libya, Minister for European Affairs Laurent Wauquiez said, criticising Tripoli for a "totally disproportionate" use of force.
But Libya has also tried to use its leverage with the European Union, warning Brussels it could suspend co-operation against illegal migration if the EU encourages pro-democracy protests.
'Tense and scary'
In a rare public admission of the unrest in Benghazi, Libyan state TV said on Sunday that an "armed people's base" in the city had come under attack and had its walls breached.
That news emerged as anti-Gaddafi activists on Twitter reported that a barracks in Benghazi had "fallen". There was no way to confirm that report.
Libya is one of several countries in the region to have seen pro-democracy campaigns since the fall of long-time Tunisian President Zine al-Abidine Ben Ali in January. Egypt's Hosni Mubarak was forced from power on 11 February.
BBC Middle East correspondent Jon Leyne says the unrest in Libya is the most serious challenge yet to Col Gaddafi.
Libyan authorities have imposed severe media restrictions.
Foreign media are largely absent from the country and internet traffic from Libya has been throttled in recent days, web analysts report.
Libyan activists opposed to Col Gaddafi, as well as Libyans operating from outside the country, are using social networks Twitter and Facebook and video-sharing site YouTube to share information and images of the protests.
But much of their information is extremely difficult to verify. Activists say they are reluctant to provide specific details of new protest locations in case security forces decide to attack their rallies.
Reports are beginning to emerge of protests in the capital, Tripoli, but information is scarce and difficult to confirm.
One woman, who gave her name as Sara, told the BBC the situation in the city was "very tense and very scary".
"Tripoli is very important, it's got people from all over Libya. Everybody's watching and waiting. Gaddafi makes us sick. This is a war," she said.
'Stop the massacre'
The doctor in Benghazi, known as Braikah, described to the BBC how casualties had been brought to the city's Jala hospital - most of them with gunshot wounds.
"Ninety per cent of these gunshot wounds [were] mainly in the head, the neck, the chest, mainly in the heart," she said.
She said the Jala hospital mortuary had 208 bodies and another hospital had 12. However, it is not clear whether all of these bodies stem from Saturday's violence.
Another doctor told the Associated Press news agency of similar numbers of bodies, but said they had been taken to the morgue since the violence began earlier in the week.
As Braikah spoke to the BBC she said new gunfire was audible within Benghazi, alleging that security forces were firing on crowds of people attempting to bury some of those killed in recent days.
Her account could not be confirmed, but many of Saturday's deaths are thought to have happened in the same way.
Witnesses said troops had used machine-guns, mortars, large-calibre weapons, and even a missile, against the mourners.
Opposition supporters said the attack was unprovoked, although security sources suggested some protesters had thrown firebombs at a government compound.
The Dubai-based Kalam institute has issued an appeal on behalf of a group of religious and clan leaders from across Libya, urging "every Muslim, within the regime" or anyone helping it [the regime]: "Do NOT kill your brothers and sisters, STOP the massacre NOW!"
Most of Benghazi is said to be controlled by anti-government protesters.
There have also been reports of anti-government protests in other eastern cities, including al-Bayda and Dernah, as well as Misrata further west, about 200km (125 miles) from the capital Tripoli.
Austria announced on Sunday that it was sending a military plane to Malta to prepare for the evacuation of Austrian and other EU nationals from Libya or other Arab countries.