Haiti's consul general to the UN, Felix Augustin, said Port-au-Prince "is flattened" and he believed more than 100,000 people were dead in the three-million population capital.
But Haitian President Rene Preval said other estimates ranged from 30,000 to 50,000, according to CNN.
Thousands of injured people waited for care outside badly damaged hospitals, while an unknown number remained trapped inside collapsed buildings.
Basic services like water and electricity were out, and the Haitian President said his government needs help clearing streets so rescuers can reach some of the hardest-hit areas.
Most hospitals, houses, schools, roads and grocery stores — virtually every necessity of basic life — has been transformed into piles of rubble.
The earthquake struck shortly before 5 p.m. Tuesday, centered about 10 miles (15 kilometers) southwest of Port-au-Prince, the U.S. Geological Survey reported. It could be felt strongly in eastern Cuba, more than 200 miles away.
The earthquake's power matched that of several nuclear bombs, said Roger Searle, a professor of geophysics in the Earth Sciences Department at Durham University in England. He said the combination of its magnitude and geographical shallowness made it particularly dangerous.
About 3 million people -- one-third of Haiti's population -- were affected by the quake, the Red Cross said. About 10 million people most likely felt shaking from the earthquake, the U.S. Geological Survey said.
"The hospitals themselves -- the destination of those patients who might survive -- they're nonexistent or have a terrible infrastructure," CNN's Dr. Sanjay Gupta reported from Port-au-Prince.
Former President Clinton, the UN special envoy to Haiti, appealed to the public to support programs that will provide food, water, shelter and medical supplies to the impoverished country, the USA Today said on its website.
"The most important thing you can do is not to send those supplies, but to send cash" to relief agencies, Clinton said.
Governments and agencies across the globe geared up to help, including rescue teams from China, Iceland and France, Haiti's onetime colonial ruler; aid flights and 3 million euros ($4.35 million) from Spain; doctors from Cuba; and a field hospital from Russia.
UN Secretary General Ban Ki-moon said the United Nations plans to release $10 million in aid immediately, while the World Bank pledged another $100 million Wednesday afternoon.
President Obama promised a "swift, coordinated and aggressive" response from the United States.
"The reports and images that we've seen of collapsed hospitals, crumbled homes and men and women carrying their injured neighbors through the streets are truly heart-wrenching," Obama said.
PA/PA
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MNA