With officials warning the overall death toll may top 100,000, there were fears that desperate Haitians, already living in one of the world's poorest nations, will soon fall prey to hunger and disease.
The stench from rows of unclaimed rotting corpses began to hang over the capital Port-au-Prince, as the international aid operation led by the United Nations and the United States swung into place.
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A planeload of 50 Chinese soldiers flew into the capital's airport at first light bringing with them three sniffer dogs, swiftly followed by two teams of French firefighters also backed by trained dogs.
"People throughout the world want to help," UN chief Ban Ki-moon said, as he also revealed that 36 UN staff had been killed in Tuesday's 7.0 quake, in the worst disaster ever for the global body.
"One of our biggest challenges will be to help them to help Haiti to the utmost," he said, adding UN members states have responded with "an outpouring of support and assistance."
Rescue teams from France, China, the United States, the Dominican Republic and Venezuela were already on the ground, he said.
Food, water and shelter are all in desperate need, as scores of people began erecting makeshift tents from sheets and awnings in a city square.
Schools, hospitals, hotels, ministries and the presidential palace lie in ruins and people caked in blood and dust pleaded for help, next to the bodies of the dead dumped in the grounds of the damaged main hospital.
A global charity warned that two million children may be at risk following the disaster as they struggle to cope alone, many sleeping next to corpses.
Reflecting the grim mood in the impoverished capital city of two million, totally unprepared to cope with a tragedy of this magnitude, a preacher warned in Creole about the end of the world.
US President Barack Obama sought to lift up a despairing people, who through the long nights since the quake have at times united in prayer and song.
"To the people of Haiti, we say clearly and with conviction, you will not be forsaken, you will not be forgotten," he said.
Washington is sending ships, helicopters, planes, rescue teams, a floating hospital and more than 5,000 troops.
The USS Carl Vinson aircraft carrier was to arrive later in the day with destroyers and more Coast Guard ships en route and 5,000 troops on stand-by.
Meanwhile, a special US rescue team from Virginia and their sniffer dogs were combing the rubble in Port-au-Prince. There were shouts of joy as one man was miraculously pulled out alive and in good shape from the remains of the UN compound where some 150 are still missing.
It was a rare moment of hope, among deepening despair.
"This is an unbelievably traumatic experience for children in Haiti," said Gareth Owen, director of emergencies for the London-based global charity Save the Children.
"No generation of Haitian children in a hundred years has experienced this scale of disaster before, and we have to act fast to minimize both the physical and psychological damage to them," Owen said.
With every hour crucial, international rescue teams were using heavy lifting gear and sniffer dogs to search the ruins, as well as distributing desperately-needed medicines, food and water.
"The priority is to find survivors," said Elisabeth Byrs, a spokeswoman for the UN's Office for the Coordination of Humanitarian Affairs. "We are working against the clock."
But the US Federal Aviation Authority said the airspace had been closed Thursday afternoon because it was saturated, and Haiti asked countries not to authorize any more flights to Port-au-Prince for now.
Casualty figures were impossible to calculate, although Haitian Prime Minister Jean-Max Bellerive said the final death toll could be "well over 100,000."
US Secretary of State Hillary Clinton, who canceled an Asia trip and returned to Washington, said the death toll would reach "tens of thousands."
The earthquake was the latest tragedy to hammer Haiti, already scarred by years of unrest, crime, political tumult and natural disaster.
Former US president Bill Clinton, a United Nations special envoy to Haiti, warned of an unprecedented humanitarian disaster.
"Early estimates indicate that nearly three million people -- almost a third of Haiti's population -- may need aid, making this one of the great humanitarian emergencies in the history of the Americas," he wrote in an op-ed published in The Washington Post.
