The tremor sparked widespread panic as it brought down buildings including the presidential palace, hotels, a hospital and the UN headquarters in the capital of Haiti, Port-au-Prince. Large numbers of UN staff were reported to be missing, as well as two American and two Australian citizens.
The quake was the strongest to hit the country in 200 years and was felt as far away as Cuba.
Thousands of people gathered in public squares late into the night, singing hymns and weeping.
Many gravely injured people sat in the streets, pleading for doctors. With almost no emergency services to speak of, the surivors had few other options.
“It’s really a catastrophe of major proportions,” Haiti’s ambassador to the United States, Raymond Alcide Joseph, told CNN television.
The death toll was expected to climb into the hundreds, but no official figure has been released so far.
“When we get an idea of the toll it will be measured in the hundreds,” a local doctor, who was bloodied and nursing an injured left arm, said. One aid worker estimated the quake had killed thousands.
Many resident were forced to abandon the ruins of their homes, and were squatting in sports grounds and open spaces in the hours following the quake. Few dared to return indoors, terrified of being buried in one of the huge aftershocks which continued to shake the city.
Rescue teams were hampered when communications went down in the minutes after the earthquake struck at 2153 GMT, but efforts free people trapped in the debris continued throughout the night.
Haitian police vehicles as well as those from the United Nations and the Red Cross tried to ferry the wounded to hospital, but progress was slow as chunks of rubble lay strewn across the roads.
Most radio and television stations stopped functioning, and the airwaves were only punctuated by a few rare radio appeals for help.
Early pictures showed scenes of utter devastation, with bodies pinned under fallen buildings as a pall of grey smoke hung over a part of one city. Dazed residents stared helplessly around them, or rushed to help the injured.
Witnesses said the damage was staggering. Tens of thousands of people are estimated to be homeless.
Rachmani Domersant, an operations manager with the Food for the Poor charity said: “There are people running, crying, screaming.”
“People are trying to dig victims out with flashlights,” he said. “I think hundreds of casualties would be a serious understatement.”
Alain Le Roy, UN peacekeeping chief, said the main UN building in Port-au-Prince had collapsed.
“We don’t know how many people were in the building,” he told reporters.
Some 200 to 250 people work in the building during normal hours. Since the earthquake struck after 5pm local time – after working hours – it was not clear how many people would have been there.