SANTO DOMINGO, Dominican Republic  A fierce earthquake struck Haiti late Tuesday afternoon, causing a crowded hospital to collapse, leveling countless shantytown dwellings and bringing even more suffering to a nation that was already the hemisphere’s poorest and most disaster-prone.

The earthquake, the worst in the region in more than 200 years, left the country in a shambles. As night fell in Port-au-Prince, Haiti’s capital, fires burned near the shoreline downtown, but otherwise the city fell into darkness. The electricity was out, telephones were not working and relief workers struggled to make their way through streets blocked by rubble.

In the chaos, it was not possible for officials to determine how many people had been killed and injured, but they warned that the casualties could be substantial.

The physical toll was easier to assess. The headquarters of the United Nations mission was seriously damaged, the United Nations said in a statement, and many employees were missing. Part of the national palace had collapsed, The Associated Press reported.

A hospital collapsed in Pétionville, a hillside district in Port-au-Prince that is home to many diplomats and wealthy Haitians, a videographer for The Associated Press said. And an American government official reported seeing houses that had tumbled into a ravine.

Tequila Minsky, a photographer based in New York who was in Port-au-Prince, said that a wall at the front of the Hotel Oloffson had fallen, killing a passer-by. A number of nearby buildings had crumbled, trapping people, she said, and a Unibank bank building was badly damaged. People were screaming.

“It was general mayhem,” Ms. Minsky said.

The earthquake, with a magnitude estimated at 7.0, struck just before 5 p.m. about 10 miles southwest of Port-au-Prince, the United States Geological Survey said. Many aftershocks followed and more were expected, said David Wald, a Geological Survey 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.”

Oxfam, an antipoverty group, said that Kristie van de Wetering, a former employee based in Port-au-Prince, had described houses in rubble everywhere.

“There is a blanket of dust rising from the valley south of the capital,” agency officials said Ms. van de Wetering had told them. “We can hear people calling for help from every corner. The aftershocks are ongoing and making people very nervous.”

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 among the most powerful 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 elsewhere in 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,” Henry Bahn, an official of the United States Department of Agriculture who was visiting Haiti, told The Associated Press. “The sky is just gray with dust.”

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, 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 United Nations peacekeeping mission that has recently reported major gains in controlling crime. International aid has also supported an array of organizations aimed at raising the country’s dismal health and education levels.

Emergency meetings were being held in Washington, and President Obama issued a statement saying that administration officials were closely monitoring the situation.

“We stand ready to assist the people of Haiti,” Mr. Obama said.

The Caribbean is not usually considered a seismic danger zone, but earthquakes have struck here in the past.

“There’s a history of large, devastating earthquakes,” said Paul Mann, a senior research scientist at the Institute for Geophysics at the University of Texas, “but they’re separated by hundreds of years.”

Most of Haiti lies on the Gonave microplate, a sliver of the earth’s crust between the much larger North American plate to the north and the Caribbean plate to the south. The earthquake on Tuesday occurred when what appears to be part of the southern fault zone broke and slid.

The fault is similar in structure to the San Andreas fault that slices through California, Dr. Mann said.

Such earthquakes, which are called strike-slip, tend to be shallow and produce violent shaking at the surface.

“They can be very devastating, especially when there are cities nearby,” Dr. Mann said.

Victor Tsai, a seismologist at the National Earthquake Information Center of the United States Geological Survey, said the depth of Tuesday’s earthquake was only about six miles and the quake was a 9 on a 1-to-10 scale that measures ground shaking. “We expect substantial damage from this event,” he said.

In the Little Haiti neighborhood of Miami, customers began streaming into the Louis Market shortly after news of the earthquake hit the airwaves. They were buying $5 phone cards in a desperate attempt to reach relatives in Haiti.

“Everyone who walks in here is crazy, worried, depressed,” said Myrlande Cherenfant, 20.

At the Notre Dame de Haiti Roman Catholic church, a handful of parishioners in red-cushioned seats pressed redial on their phones over and over. Some said that they had been able to get through immediately after the earthquake.

“I was able to talk to a priest in Haiti,” the Rev. Reginald Jean-Mary said. “The only word I heard was ‘catastrophe’ and then it cut off.”

He said that in a later call he was told that the cathedral in Port-au-Prince had been destroyed and that other churches had been damaged.

Jean-Robert Lafortune, executive director of the Miami-based Haitian American Grassroots Coalition, said that Haiti had endured “a cycle of natural disasters and man-made disasters, and this is one more big catastrophe.”

“We are in trauma,” he said. “We have loved ones there and many of them will be victims. We’re calling and calling, but there’s nothing on the other end.”

Simon Romero reported from Santo Domingo, and Marc Lacey from Mexico City. Reporting was contributed by Elisabeth Malkin from Mexico City, Damien Cave from Miami, and Kenneth Chang and Liz Robbins from New York.