The streets of Libya's capital are littered with the bodies of scores of protesters shot dead by security forces loyal to Muammar Gaddafi, an opposition leader said today.
Mohammed Ali of the Libyan Salvation Front said Tripoli's inhabitants are hiding at home after the killings and warnings by forces loyal to Gaddafi that anyone found outside would be shot.
Mr Ali said Gaddafi's forces shot at ambulances and some protesters were left bleeding to death.
The UN Office of the High Commissioner for Human Rights said at least 250 people have been killed and hundreds more injured.
Human Rights Watch has put the toll at at least 233 killed. The difficulty in getting information made obtaining a precise figure impossible.
The head of the UN agency, Navi Pillay, called for an investigation, saying widespread and systematic attacks against the civilian population "may amount to crimes against humanity."
World leaders also have expressed outrage. US Secretary of State Hillary Clinton called on Gaddafi to "stop this unacceptable bloodshed" and said the world was watching the events "with alarm."
Mr Ali said scores of bodies had been left on the streets in the Fashloum district after the pro-Gaddafi gunmen opened fire the night before.
Gaddafi, the longest serving Arab leader, appeared briefly on TV early today to dispel rumours that he had fled. Sitting in a car in front of what appeared to be his residence and holding an umbrella out of the passenger side door, he told an interviewer that he had wanted to go to the capital's Green Square to talk to his supporters, but the rain stopped him.
"I am here to show that I am in Tripoli and not in Venezuela. Don't believe those misleading dog stations," he said, referring to the media reports that he had left the country.
The video clip and comments lasted less than a minute - unusual for the mercurial leader, who is known for rambling speeches that often last hours.
Pro-Gaddafi militia drove through Tripoli with loudspeakers and told people not to leave their homes as security forces sought to keep the unrest that swept eastern parts of the country - leaving the second-largest city of Benghazi in protesters' control - from overwhelming the capital of two million people.
Protesters called for a demonstration in Tripoli's central Green Square and in front of Gaddafi's residence, but witnesses in various areas described a scene of intimidation: helicopters hovering above the main seaside boulevard and pro-Gaddafi gunmen firing from moving cars and even shooting at the facades of homes to terrify the population.
Youths trying to gather in the streets scattered and ran for cover amid gunfire.
Gaddafi appeared to have lost the support of at least one major tribe, several military units and his own diplomats, including Libya's ambassador in Washington, Ali Adjali. Deputy UN Ambassador Ibrahim Dabbashi accused him of committing genocide against his own people in the current crisis.
UN Secretary-General Ban Ki-moon described the crackdown as "a serious violation of international humanitarian law."
The chaos engulfing the country prompted many foreigners to flee.
Italy sent an air force jet to Benghazi, Libya's second-largest city, to evacuate around 100 Italian citizens. Many countries had already urged their nationals to avoid non-essential travel to Libya, or recommended that those already there leave on commercial flights.
Egyptian troops, meanwhile, have strengthened their presence on the border with Libya and set up a field hospital as thousands of Egyptians return home from Libya by land.
Oil companies, including Italy's Eni, Shell and BP have also begun evacuating their expat workers or their families or both.
State TV quoted Gaddafi's son, Saif al-Islam, as saying the military conducted airstrikes on remote areas, away from residential districts, on munitions warehouses, denying reports that warplanes attacked Tripoli and Benghazi.