PORT-AU-PRINCE: More than 100,000 people were feared dead Wednesday after a cataclysmic earthquake struck Haiti, filling the streets with corpses and burying thousands under razed schools, hotels and hospitals.

Desperate victims pleaded for help from beneath mountains of concrete and the capital Port-au-Prince was "mostly destroyed" an AFP correspondent said, as a humanitarian disaster loomed for survivors facing a second night outdoors.

In horrific scenes, some injured lay in the back of a pick-up truck, a dead victim was seen pinned between the fallen roof of her home and her bed, and rescuers tore at the wreckage of a children's hospital with their bare hands.

With every hour crucial for those trapped, a massive aid operation swung into action, with rescue teams flying in from across the globe, bringing heavy lifting gear and desperately-needed medicines and food.

Casualty figures were impossible to calculate, but Prime Minister Jean-Max Bellerive told CNN the final death toll from the 7.0 quake could be "well over 100,000." President Rene Preval told the network 50,000 could be dead.

Bodies line Port-au-Prince streets as dusk falls

US Secretary of State Hillary Clinton compared the tragedy to the Asian tsunami which killed more than 220,000 people five years ago.

"The Indian Ocean tsunami was such a terrible tragedy and with such high loss of life. This will be a very high loss of life as well," she said.

As people clawed fallen masonry to search for bloodied survivors, Preval painted a scene of utter devastation.

"Parliament has collapsed. The tax office has collapsed. Schools have collapsed. Hospitals have collapsed," he told the Miami Herald.

With thousands of people missing, dazed survivors in torn clothes wandered through the rubble as more than 30 aftershocks rocked the ramshackle capital, where more than two million people live, most in extreme poverty.

Dust filled the air, scattered fires broke out, and injured people slumped on the blood-soaked floor of one clinic waiting for treatment. Elsewhere outside a field hospital, mothers huddled with shell shocked children.

Some injured survivors wore makeshift slings and blood-soaked bandages. Others were carried on bits of debris used as stretchers, past piles of smashed concrete, from which crushed bodies protruded.

Fanning safety fears in the crime-hit capital, the United Nations said the main prison had collapsed, allowing some inmates to flee into a city where basic services and communications were shut down.

The earthquake was the latest tragedy to hammer Haiti, which has been scarred by years of unrest, crime and political tumult.

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"They have had a long and tortured history, but they are good people, they are survivors," said former US president Bill Clinton, the UN envoy to Haiti.

"These people deserve a chance to bury their dead, to heal their wounded, to eat, to sleep, to begin to recover, and they can't do it with just government help alone," Clinton said, appealing on CNN for cash to buy food aid.

The quake late Tuesday struck just below the earth's surface on a notorious fault line, meaning the shock was intense and damage severe, scientists said.

Preval said that head of the UN mission to Haiti Hedi Annabi was killed when the UN headquarters collapsed.

UN officials could not confirm that, but said at least 14 of its staffers were dead, 56 were injured and a further 150 were missing.

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Preval's wife, First Lady Elisabeth Preval, told the Miami Herald she had seen bodies in the streets of Port-au-Prince and had heard the cries of victims still trapped in the rubble of the parliament building.

"I'm stepping over dead bodies. A lot of people are buried under buildings. The general hospital has collapsed. We need support. We need help. We need engineers," she said.

US President Barack Obama vowed a swift and aggressive effort to save lives and spoke with an array of regional leaders to coordinate the aid operation.

"This tragedy seems especially cruel and incomprehensible," he said.

Officials said the first US search and rescue teams workers were already deploying from the airport in Port-au-Prince.

The US military also mobilized ships and aircraft and an aircraft carrier, the USS Carl Vinson, was due to arrive Thursday.

Officials said the naval base in Guantanamo Bay, Cuba, home to a controversial camp for terror detainees, may also be used to house refugees.

Hillary Clinton meanwhile cut short a trip to Asia to return to Washington to help deal with the crisis.

Plane-loads of rescue teams and relief supplies were quickly dispatched from nations including Britain, Canada, Russia, Spain, France, Germany, the Netherlands and Russia.

The World Bank said it planned to extend an additional 100 million dollars in emergency aid to Haiti to help recovery and reconstruction.

The massive quake toppled the cupola on the gleaming white presidential palace, a major hotel where 200 tourists were missing and the headquarters of the UN mission in Haiti.

Jordan reported that three of its peacekeepers were killed and 21 wounded in the quake. Brazil said 11 of its peacekeepers were killed while eight Chinese soldiers were buried in rubble and 10 were missing, state media said.

The Haitian resort town of Jacmel was also devastated.

"I was driving back to Jacmel in the mountains when the entire mountain seemed to fall down all around me," said Emmet Murphy, local head of the US non-governmental organization ADCI/VOCA.

Two hundred foreigners were missing at the Hotel Montana, French Secretary of State for Cooperation Alain Joyandet said.