PORT-AU-PRINCE, Haiti — Survivors strained desperately on Wednesday against the chunks of concrete that buried this city along with thousands of its residents, rich and poor, from shantytowns to the presidential palace, in the devastating earthquake that struck late Tuesday afternoon.

Calling the death toll “unimaginable” as he surveyed the wreckage, Haiti’s president, René Préval, said he had no idea where he would sleep. Schools, hospitals and a prison collapsed. Sixteen United Nations peacekeepers were killed and at least 140 United Nations workers were missing, including the chief of its mission, Hédi Annabi. The city’s archbishop, Msgr. Joseph Serge Miot, was feared dead.

And the poor who define this nation squatted in the streets, some hurt and bloody, many more without food and water, close to piles of covered corpses and rubble. Limbs protruded from disintegrated concrete, muffled cries emanated from deep inside the wrecks of buildings — many of them poorly constructed in the first place — as Haiti struggled to grasp the unknown toll from its worst earthquake in more than 200 years.

In the midst of the chaos, no one was able to offer an estimate of the number of people who had been killed or injured, though there was widespread concern that there were likely to be thousands of casualties.

“Please save my baby!” Jeudy Francia, a woman in her 20s, shrieked outside the St.-Esprit Hospital in the city. Her child, a girl about 4 years old, writhed in pain in the hospital’s chaotic courtyard, near where a handful of corpses lay under white blankets. “There is no one, nothing, no medicines, no explanations for why my daughter is going to die.”

Image Credit... The New York Times

Governments and aid agencies from Beijing to Grand Rapids began marshaling supplies and staffs to send here, though the obstacles proved frustrating just one day after the powerful 7.0-magnitude earthquake hit. Power and phone service were out. Flights were severely limited at Port-au-Prince’s main airport, telecommunications were barely functioning, operations at the port were shut down and most of the medical facilities had been severely damaged, if not leveled.

A Red Cross field team of officials from several nations had to spend Wednesday night in Santo Domingo in the Dominican Republic to gather its staff before taking the six-hour drive in the morning across the border to the earthquake zone.

“We were on the plane here with a couple of different agencies, and they all are having similar challenges of access,” Colin Chaperon, a field director for the American Red Cross, said in a telephone interview. “There is a wealth of resources out there, and everybody has the good will to go in and support the Haitian Red Cross.”

The quake struck just before 5 p.m. on Tuesday about 10 miles southwest of Port-au-Prince, Haiti’s capital, ravaging the infrastructure of Haiti’s fragile government and destroying some of its most important cultural symbols.

“Parliament has collapsed,” Mr. Préval told The Miami Herald. “The tax office has collapsed. Schools have collapsed. Hospitals have collapsed. There are a lot of schools that have a lot of dead people in them.”

He added: “All of the hospitals are packed with people. It is a catastrophe.”

President Obama promised that Haiti would have the “unwavering support” of the United States.