Looting began almost as quickly as the quake struck at 4:53 pm and people were seen carrying food from collapsed buildings. Many lugged what they could salvage and stacked it around them as they slept in streets and parks.

People streamed into the Haitian countryside, where wooden and cinderblock shacks showed little sign of damage. Many balanced suitcases and other belongings on their heads. Ambulances and UN trucks raced in the opposite direction, toward Port-au-Prince.

A small contingent of US ground troops could be on their way soon, although it was unclear whether they would be used for security operations or humanitarian efforts. Gen. Douglas Fraser, commander of the US Southern Command, said roughly 2,000 Marines as part of an expeditionary unit might be deployed aboard a large-deck amphibious ship. Fraser said the ship could provide medical help.

Other nations -- from Iceland to Venezuela -- said they would start sending in aid workers and rescue teams. Cuba said its existing field hospitals in Haiti had already treated hundreds of victims. The United Nations said Port-au-Prince's main airport was "fully operational" and open to relief flights.

The US Navy aircraft carrier, USS Carl Vinson, is under way and expected to arrive off the coast of Haiti Thursday. Additional US Navy ships are under way to Haiti, a statement from the Southern Command said.

Aftershocks continued to rattle the capital of 2 million people as women covered in dust clawed out of debris, wailing. Stunned people wandered the streets holding hands. Thousands gathered in public squares to sing hymns.

UN humanitarian chief John Holmes said it was possible that the death toll "will be in the thousands."

"Initial reports suggest a high number of casualties and, of course, widespread damage but I don't have any figure that I can give you with any reliability of what the number of casualties will be," Holmes said.

People pulled bodies from collapsed homes, covering them with sheets by the side of the road. Passers-by lifted the sheets to see if loved ones were underneath. Outside a crumbled building, the bodies of five children and three adults lay in a pile.

The prominent died along with the poor: the body of Archbishop Joseph Serge Miot, 63, was found in the ruins of his office, said the Rev. Pierre Le Beller of the Saint Jacques Missionary Center in Landivisiau, France. He told The Associated Press by telephone that fellow missionaries in Haiti had told him they found Miot's body.

Preval told the Herald that Haiti's Senate president was among those trapped alive inside the Parliament building. Much of the National Palace pancaked on itself.

The international Red Cross and other aid groups announced plans for major relief operations in the Western Hemisphere's poorest country.

Tens of thousands of people lost their homes as buildings that were flimsy and dangerous even under normal conditions collapsed. Nobody offered an estimate of the dead, but the numbers were clearly enormous.

"The hospitals cannot handle all these victims," said Dr. Louis-Gerard Gilles.

Medical experts say disasters such as an earthquake generally do not lead to new outbreaks of infectious diseases, but they do tend to worsen existing health problems.

Haiti's quake refugees likely will face an increased risk of dengue fever, malaria and measles -- problems that plagued the impoverished country before, said Kimberley Shoaf, associate director of the UCLA Center for Public Health and Disasters.

Some of the biggest immediate health threats include respiratory disease from inhaling dust from collapsed buildings and diarrhea from drinking contaminated water.

With hospitals and clinics severely damaged, Haiti will also face risks of secondary infections. People seeking medical attention for broken bones and other injuries may not be able to get the help they need and may develop complications.