The quake had a magnitude of 7.0, according to the U.S. Geological Survey, and was centered just 10 miles from Port-au-Prince.
The center was also relatively shallow, less than 10 miles below ground, increasing the risk of damage.
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," a spokeswoman for the aid group told The Associated Press.
"He reported that it was just total disaster and chaos, that there were clouds of dust surrounding Port-au-Prince," Sara Fajardo said from the group's offices in Maryland.
State Department Asst. Secretary for Public Affairs P.J. Crowley told reporters that embassy staff "reported structures down. They reported a lot of walls down. They did see a number of bodies in the street and on the sidewalk that had been hit by debris. So clearly, there's going to be serious loss of life in this."
Early reports said a hospital in nearby Petionville had collapsed, and a videographer for The Associated Press said he could hear people screaming for help.
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."
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 as he drove 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: "For this to come and hit us is like a ouble whammy."
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."
White House and State Department Offer Help
The White House issued a statement tonight from President Obama: "My thoughts and prayers go out to those who have been affected by this earthquake. We are closely monitoring the situation and we stand ready to assist the people of Haiti."
U.S. military and humanitarian services said they were ready to offer help, but officials said privately that they had to wait for a formal request from the Haitian government.
"The United States is offering our full assistance to Haiti and to others in the region. We will be providing both civilian and military disaster relief and humanitarian assistance and our prayers are with the people who have suffered, their families and their loved ones," Secretary of State Hillary Clinton said in Honolulu, Hawaii, where she was to make a speech on Asian relations.
"We are standing by to help in any way we can," said Crowley.
The U.S. Agency for International Development asked California Task Force 2, the Los Angeles County Fire Departent's search and rescue team, and Virginia Task Force 1, sponsored by the Fairfax County, Va., Fire and Rescue Department, to prepare to deploy to Haiti to help with the recovery effort, should the country request assistance.
The Coast Guard has said that it will send out reconnaissance planes at first light tomorrow to assess the damage to the region. They also have four cutters in the area should a request be made for assistance.
Efforts to get aid into Haiti could be hampered by damage to the airport in Port-au-Prince. The Federal Aviation Administration reported that the air traffic control tower at the airport has collapsed.
Flights were being routed around Haitian air space Tuesday night because no air traffic control services were being provided.
Earthquake in Haiti: 7.0 Magnitude
"Everybody is just totally, totally freaked out and shaken," said Henry Bahn, a visiting official with the U.S. Department of Agriculture. "The sky is just gray with dust."
Bahn said he was walking to his hotel room when the ground began to shake.
"I just held on and bounced across the wall," he said. "I just hear a tremendous amount of noise and shouting and screaming in the distance."
In the United States, Rose Leandre was anxiously trying to get through to any family members in her native Haiti on Tuesday evening.
The Spring Valley, N.Y., woman runs the Haitian-American Cultural and Social Organization, which provides immigration, social and educational outreach to the sizable Haitian community --- estimated to be at least 11,000, according to U.S. Census figures -- in Rockland County, one of New York City's northern surburbs.
"Everybody's been calling all over and calling each other, 'have you heard from your family?' And so far, no one's getting through … so far, within the community, no one has gotten through to anyone in Haiti," Leandre said.
Leandre added that she was desperate to make contact with her mother, who traveled to Haiti, to an area 20 minutes outside Port-au-Prince, just Monday.
"She's disabled, she has severe arthritis, so during the wintertime, it's horrible for her, so she usually goes and spends a month or two in warmer weather," Leandre said.
Leandre said the community would likely meet Wednesday to discuss relief efforts.
A tsunami watch was issued by the Pacific Tsunami Warning Center, but it was canceled after several hours.
"There could have been destructive tsunami waves near the earthquake epicenter but there is not a threat to coastal areas further away," said an advisory from the warning center.
Haiti is the poorest country in the western hemisphere, with a population estimated at 9.8 million people, according to the World Bank.
ABC News Radio and The Associated Press contributed to this report.