A Reuters reporter, quoting workers for a U.S. charity, Food for the Poor, said there were more houses destroyed than standing in Delmas Road, a major thoroughfare in Port-au-Prince. An employee of Food for the Poor said a five-story building had collapsed.
There were other reports of damage, but they could not be immediately confirmed.
"I couldn't even stand up, that's how bad it was," said Valerie Moliere, a 15-year-old resident of Port-au-Prince, reached by ABC News. "There's a lot of people in the street everywhere. Some are wounded."
"I just heard that right next to my neighborhood there's this pharmacy and this school that broke down and many people died," she said.
"I don't know how powerful it was. But from what I felt, it was very powerful," said Carole Bastin, another resident reached by ABC News. "And it lasted like ... I don't know maybe, it was quite long because I could not walk, I was trying to leave the house, the building, I could not because everything was shaking around me. All the file cabinets were opening and all the things falling apart."
The earthquake was felt by people in Jamaica; Guantanamo Bay, Cuba; and Santo Domingo in the Dominican Republic.
The original earthquake was quickly followed by two strong aftershocks. The USGS said they had initial magnitudes of 5.9 and 5.5, and more were likely.
"We've seen this with almost all large, shallow earthquakes," said Don Blakeman of the National Earthquake Information Center in a telephone interview with ABC News. "We're going to have an aftershock series, and as time passes, the size of the aftershocks and the frequency will decrease."
Carel Pedre, a Haitian journalist speaking to French digital channel iTele, said there had been several aftershocks. In an emotional interview, he said, "I've not had any contact from my mother and my brother and I wonder about what's going to happen and I'm scared."
"We had an earthquake at 4 o'clock, at 5 o'clock and now it's midnight and there are still aftershocks and we're weak, we don't have the means with which to react, we can't get everyone out of here and we don't have enough food for everyone. There are people out there dying," he said.
Haiti's ambassador to the U.S., Raymond Joseph, said he spoke with President Rene Preval, who told him that buildings were collapsing around him while driving his car.
"The president is fine and [the first lady] is fine," he said. "However, the palace building has been damaged."
The ambassador added that Haiti endured four hurricanes in 2008 and that "for this to come and hit us is like a double whammy."
Boston-based non-profit Partners in Health (PIH), which has offices in Haiti, released a statement saying "The earthquake has destroyed much of the already fragile and overburdened infrastructure in the most densely populated part of the country. A massive and immediate international response is needed to provide food, water, shelter, and medical supplies for tens of thousands of people."
Louise Ivers, PIH clinical director in Haiti, made an urgent appeal for help in an e-mail to colleagues. "Port-au-Prince is devastated, lot of deaths. SOS. SOS... Temporary field hospital by us at UNDP needs supplies, pain meds, bandages. Please help us."