A fierce earthquake struck the star-crossed country of Haiti late yesterday afternoon, causing a crowded hospital to collapse, leveling countless shantytown dwellings and bringing even more suffering to a place that was already the hemisphere's poorest and most disaster-prone spot.

A reporter from the Reuters news agency said he had seen dozens of dead and injured people in the rubble in Port-au-Prince, Haiti's capital. Many of the city's streets were blocked.

"Everything started shaking, people were screaming, houses started collapsing," said the reporter, Joseph Guyler Delva. "It's total chaos. I saw people under the rubble, and people killed."

As night fell in Haiti, there was no estimate of how many people had been killed and injured, but officials warned that casualties would be substantial.

The powerful earthquake, with a magnitude estimated at 7.0, struck just before 5 p.m. about 10 miles west of Port-au-Prince, the U.S. Geological Survey said.

Five aftershocks followed and more were expected, said David Wald, a USGS seismologist. "The main issue here will probably be shaking," he said, "and this is an area that is particularly vulnerable in terms of construction practice, and with a high population density. There could be a high number of casualties."

The earthquake could be felt across the border in the Dominican Republic, on the eastern part of the island of Hispaniola. High-rise buildings in the capital, Santo Domingo, shook and sent people streaming down stairways into the streets, fearing that the tremor could intensify.

Haiti sits on a large fault that has caused catastrophic quakes in the past, but this one was described as the most powerful ever to hit the region.

With many poor residents living in tin-roof shacks that sit precariously on steep ravines and with much of the construction in Port-au-Prince and throughout the country of questionable quality, the expectation was that the quake caused major damage to buildings and significant loss of life.

"Everybody is just totally, totally freaked out and shaken," said Henry Bahn, an official of the U.S. Department of Agriculture. Haiti's many man-made woes -- its dire poverty, political infighting and proclivity for insurrection -- have been exacerbated repeatedly by natural disasters.

At the end of 2008, a series of four hurricanes flooded whole towns, knocked out bridges and left a destitute population in even more desperate conditions.

The United States and other countries have devoted significant humanitarian support to Haiti, financing a large U.N. peacekeeping mission that has recently reported major gains in controlling crime.

The U.N. reported that its headquarters was badly damaged and that many members of the force were unaccounted for.

Victor Tsai, a seismologist at the National Earthquake Information Center of the U.S. Geological Survey, said the depth of the earthquake was only about six miles and the quake was a 9 on a 1-to-10 scale that measures ground shaking.