A Food for the Poor charity employee said there were more houses destroyed than standing in Delmas Road, a major thoroughfare in the capital.
"The whole city is in darkness, you have thousands of people sitting in the streets, with nowhere to go," Rachmani Domersant, the charity's operations manager, said.
"I've seen seven to eight buildings, from office buildings to hotels and shopping stores, collapsed ... I think hundreds of casualties would be a serious understatement."
The presidential palace in the capital was among the buildings badly damaged, Raymond Alcide Joseph, Haiti's ambassador to the US, said.
Joseph told Al Jazeera that he had spoken to Fritz Longchamp, chief of staff to Haiti's president, who said "buildings were crumbling right and left" near the palace.
"I think it's really a catastrophe of major proportions," Joseph said.
Hospitals, schools and hotels collapsed in the capital, raising fears that the injured would have nowhere to go to get treatment.
Television footage showed buildings long cracks in buildings that were still standing.
The United Nations headquarters in the capital was also reported to be severely damaged and many of its staff were missing.
"The United Nations can confirm that the Headquarters of the United Nations Stabilisation Mission in Haiti (MINUSTAH) in Port-au-Prince has sustained serious damage along with other UN installations," Alain le Roy, the under-secretary-general for peacekeeping operations, said.
The Caribbean nation of nine million is the poorest country in the Americas with an annual per-capita income of $560. It ranks 146th out of 177 countries on the UNDP Human Development Index
More than half the population lives on less than $1 a day and 78 per cent on less than $2. There is a high infant mortality rate and the prevalence of HIV among those between ages 15 and 49 is 2.2 per cent
Haiti's infrastructure is close to total collapse and severe deforestation has left only two per cent of forest cover
After decades of dictatorship, former Roman Catholic priest Jean-Bertrand Aristide became Haiti's first freely elected leader in 1990.
He was ousted by a military coup in 1991 but reinstated with US backing. He was forced out of the country and into exile in 2004 by a rebellion of gangs and former soldiers
Haiti has been led by Rene Preval since May 2006, when the country returned to constitutional rule
About 9,000 UN police and troops are stationed in the country to maintain order
"For the moment, a large number of personnel remain unaccounted for," he said in a statement issued in New York.
The magnitude 7.0 quake's epicentre was about eight to 10km deep, a relatively shallow depth which was likely to have magnified the destruction, according to seismologists.
The quake, which was followed by several aftershocks up to 5.9 in magnitude, prompted a tsunami alert for parts of the Caribbean that was later cancelled.
Patrick McCormick, a spokesman for Unicef, told Al Jazeera that the quake had created a "massive emergency" and the UN children's agency was "looking at bringing supplies especially water, medicine and shelter to those affected".
Barack Obama, the US president, said his thoughts and prayers were with the people of Haiti and the US was ready to help the island nation.
Hillary Clinton, Obama's secretary of state, said the US would provide civilian and military disaster relief assistance.
Tuesday's quake was felt as far away as southeastern Cuba, about 257km from the epicentre, prompting Cuban authorities to evacuate coastal residents because of the initial tsunami threat.
Soldiers at the US naval base at Guantanamo Bay in eastern Cuba also felt the quake but there was no damage to the base or the prison where the US holds about 200 foreign detainees.
Chief Petty Officer Bill Mesta, a sailor at the base, said troops had begun checking stockpiles of blankets, tents and other relief supplies in anticipation that they will be asked to help in the relief effort.
The last major earthquake that hit Haiti - a magnitude 6.7 temblor – struck in 1984.
Already the poorest nation in the Americas, Haiti has been hit by a series of disasters recently and was battered by hurricanes in 2008.