The Haitian capital has largely been destroyed in the most powerful earthquake to hit the country in more than 200 years.
Journalists from The Associated Press describe severe and widespread casualties after touring the streets of Port-au-Prince.
The damage is staggering even in a country accustomed to tragedy and disaster. AP reporters said the National Palace is a crumbled ruin and tens of thousands of people are homeless.
Many gravely injured people sit in the street, pleading for doctors many hours after the quake. In public squares thousands of people are singing hymns and holding hands.
The 7.0-magnitude quake struck at 4:53 p.m. Tuesday, leaving large numbers of people unaccounted for.
An aid official described "total disaster and chaos."
Communications were widely disrupted, making it impossible to get a clear picture of damage as powerful aftershocks shook a desperately poor country where many buildings are flimsy. Electricity was out in some places.
Karel Zelenka, a Catholic Relief Services representative in Port-au-Prince, told U.S. colleagues before phone service failed that "there must be thousands of people dead," according to a spokeswoman for the aid group Sara Fajardo.
"He reported that it was just total disaster and chaos, that there were clouds of dust surrounding Port-au-Prince," Fajardo said from the group's offices in Maryland.
The headquarters of the UN peacekeeping Mission in Haiti sustained "serious damage" in Tuesday's earthquake and a large number of UN personnel in Haiti are missing, the peacekeeping chief said late Tuesday.
Alain Le Roy in New York said other UN installations in the Caribbean nation were also damaged.
The executive director of Haitian Ministries for the Diocese of Norwich, Conn., Emily Smack, said she believed two of the organization's staff are trapped in their mission house, which partially collapsed during the earthquake.
Other buildings also were damaged and scientists said they expected "substantial" damage and casualties as powerful aftershocks shook the country.
The earthquake had a magnitude of 7.0 and was centred about 15 kilometres west of Port-au-Prince, the U.S. Geological Survey said.
It had a depth of eight kilometres and was the largest quake recorded in the area, said Dale Grant, an analyst with the U.S. agency. The last major quake was a magnitude-6.7 temblor in 1984.
Weather conditions were generally quiet at the time of the quake. A mostly sunny sky with temperatures around 25 C to 30 C were reported.
An Associated Press videographer saw the wrecked hospital in Petionville, a hillside Port-au-Prince district that is home to many diplomats and wealthy Haitians. Elsewhere, a U.S. government official reported seeing houses that had tumbled into a ravine.
Raymond Joseph, Haiti's ambassador to the U.S., said from his Washington, D. C., office that he spoke to President Rene Preval's chief of staff, Fritz Longchamp, just after the quake hit.
He said Longchamp told him that "buildings were crumbling right and left" near the national palace. He said he has not been able to get through by phone to Haiti since.
Don Blakeman, an analyst at the USGS in Golden, Colo., said such a strong quake carried the potential for widespread damage.
"I think we are going to see substantial damage and casualties," he said. The quake was felt in the Dominican Republic, which shares a border with Haiti on the island of Hispaniola. Some panicked residents in the capital of Santo Domingo fled from their shaking homes.
In eastern Cuba, houses shook but no major damage was immediately reported.
"We felt it very strongly and I would say for a long time," said Msgr. Dionisio Garcia, archbishop of Santiago. "We had time to evacuate."