More than 100,000 people were feared dead in Haiti Wednesday after a calamitous earthquake razed homes, hotels, and hospitals, leaving the capital in ruins and bodies strewn in the streets.

Schools collapsed, trapping the dead inside, and the cries of desperate victims escaped from flattened buildings in the center of the capital Port-au-Prince, which an AFP correspondent said was "mostly destroyed."

A massive aid operation swung into action, with rescue teams set to fly in from across the globe to try to pull victims from the debris, bringing desperately-needed medicines and food, as a humanitarian crisis unfolded.

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.

As people clawed through 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.

Some injured survivors wore makeshift slings and blood-soaked bandages. One woman was carried on a bit of debris used as a stretcher, past piles of smashed concrete, from which crushed bodies protruded.

A second long night in the open beckoned for tens of thousands of people with nowhere to sleep, and no tools but their bare hands to try to rescue trapped compatriots.

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.

"It is biblical, the tragedy that continues to stalk Haiti and the Haitian people," said US Secretary of State Hillary Clinton, as she said she would shorten an Asian tour to deal with the crisis.

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.

With Haitian hospitals also having crumbled in the fury of the quake, medical services were struggling to cope with the flow of wounded.

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.

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.

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.

An Argentine-staffed hospital was the only one left operating in the city and was struggling to cope with huge numbers of injured, its director told Argentine television.

"The situation is really critical because we cannot cope with this many dead and injured," Daniel Desimone told Todo Noticias.

"There are a lot of dead people in the streets, a lot of injured," he added.

US President Barack Obama vowed a swift and aggressive effort to save lives and said search and rescue teams would arrive within hours after a "heart wrenching" earthquake.

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

The US military mobilized ships, aircraft and expert teams due to arrive within hours to help the relief effort. 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.

Planeloads 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 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.