A 7.0-magnitude quake which hit south of the Haitian capital Port-au-Prince is feared to have killed hundreds of people across the Caribbean country.
In the space of a minute, Haiti's worst quake in two centuries wrecked the HQ of the UN mission, the presidential palace and numerous other buildings.
A "large number" of UN personnel were reported missing by the organisation.
The capital is now said to be in total darkness with many people sleeping outside amid fear of more aftershocks.
Describing the earthquake as a "catastrophe", Haiti's envoy to the US said the cost of the damage could run into billions.
The quake, which struck about 15km (10 miles) south-west of Port-au-Prince, was quickly followed by two strong aftershocks of 5.9 and 5.5 magnitude.
The tremor hit at 1653 (2153 GMT), the US Geological Survey said. Phone lines to the country failed shortly afterwards.
Aid workers and reporters at the scene estimated the number of dead to be in the hundreds or even thousands.
As night fell, Rachmani Domersant, an operations manager with the Food for the Poor charity, told Reuters: "The whole city is in darkness.
"You have thousands of people sitting in the streets with nowhere to go. There are people running, crying, screaming."
Adding that he had seen no rescue vehicles in the hillside suburb of Petionville, he said: "People are trying to dig victims out with flashlights.
"I think hundreds of casualties would be a serious understatement."
There have been some reports of looting overnight.
Earlier, bodies white with dust could be seen piled on the back of a pick-up truck as vehicles tried to ferry the injured to hospital.
Haiti is the poorest country in the western hemisphere and has suffered a number of recent disasters, including four hurricanes and storms in 2008 that killed hundreds.
'Three million affected'
In a statement issued in New York, the UN said that its local HQ in Haiti had "sustained serious damage along with other UN installations" and "a large number" of personnel were missing.
UN peacekeeping chief Alain Le Roy said it was unclear how many people had been inside the building.
The head of the UN mission in Haiti, Hedi Annabi, was reported to have been in the building and is unaccounted for.
The UN's stabilisation mission plays a vital role in ensuring security in Haiti.
Raymond Joseph, Haiti's ambassador to the US, said the presidential palace, the tax office, the ministry of commerce and the foreign ministry had all been damaged, but the airport was intact.
He and Haiti's ambassador to Mexico, Robert Manuel, both said that President Rene Preval and his wife had survived the quake.
The World Bank, which said its local offices were destroyed but that most staff were accounted for, planned to send a team to help Haiti assess damage and plan a recovery, Reuters reported.
Mike Blanpied of the US Geological Survey said that, based on the location and size of the quake, about three million people would have been severely shaken by its impact.
"This quake occurred under land as opposed to offshore, so a lot of people were directly exposed to the shaking coming off that earthquake fault, which was quite shallow," he told the BBC.
US President Barack Obama said in a statement that his "thoughts and prayers" were with the people of Haiti and America stood ready to assist them.
UN Secretary General Ban Ki-moon said: "My heart goes out to the people of Haiti after this devastating earthquake."
'Rubble and wire'
In the minutes after the quake, Henry Bahn, a visiting official from the US Department of Agriculture, said he had seen houses which had tumbled into a ravine.
"Everybody is just totally, totally freaked out and shaken," said Mr Bahn, who described the sky as "just grey with dust".
He said he had been walking to his hotel room when the ground began to shake.
"I just held on and bounced across the wall," he said. "I just heard a tremendous amount of noise and shouting and screaming in the distance."
He said rocks were strewn all over the place, and the ravine was "just full of collapsed walls and rubble and barbed wire".
BBC News website readers in the Dominican Republic, which borders Haiti on the island of Hispaniola, said they had also felt the quake.
"My family is on the 8th floor of a tower in downtown Santo Domingo," wrote Max Levine.
"We felt a swaying of the building for 5-10 seconds. All the lamps were swinging. There was a 20-second pause and then another similar sway. We rushed out of the building with many others to the street."
In the immediate aftermath of the quake, a tsunami watch was put out for Haiti, Cuba and the Bahamas, but this was later lifted.