PORT-AU-PRINCE,Haiti —    Dazed survivors wandered past dead bodies in rubble-strewn streets Wednesday, crying for loved ones, and rescuers searched collapsed buildings as officials feared the death toll from Haiti’s devastating earthquake could reach into the tens of thousands.

The first cargo planes with food, water, medical supplies, shelter and sniffer dogs headed to the Western Hemisphere’s poorest nation a day after the magnitude – 7 quake flattened much of the capital of 2 million people.

Tuesday’s earthquake brought down buildings great and small — from shacks in shantytowns to President Rene Preval’s gleaming white National Palace, where a dome tilted ominously above the manicured grounds.

Hospitals, schools and the main prison collapsed. The capital’s Roman Catholic archbishop was killed when his office and the main cathedral fell. The head of the U.N. peacekeeping mission was missing in the ruins of the organization’s multistory headquarters.

At a triage center improvised in a hotel parking lot, people with cuts, broken bones and crushed ribs moaned under tent-like covers fashioned from bloody sheets.

“This is much worse than a hurricane,” said doctors’ assistant Jimitre Coquillon. “There’s no water. There’s nothing. Thirsty people are going to die.”

Bodies were everywhere in Port-au-Prince: those of tiny children adjacent to schools, women in the rubble-strewn streets with stunned expressions frozen on their faces, men hidden beneath plastic tarps and cotton sheets.

Haiti’s leaders struggled to comprehend the extent of the catastrophe — the worst earthquake to hit the country in 200 years — even as aftershocks still reverberated.

“It’s incredible,” Preval said to CNN. “A lot of houses destroyed, hospitals, schools, personal homes. A lot of people in the street dead. … I’m still looking to understand the magnitude of the event and how to manage.”

Preval said thousands of people were probably killed. Leading Sen. Youri Latortue told The Associated Press that 500,000 could be dead, but conceded that nobody really knows.

“Let’s say that it’s too early to give a number,” Preval said.

Haiti seems especially prone to catastrophe — from natural disasters like hurricanes, storms, floods and mudslides to crushing poverty, unstable governments, poor building standards and low literacy rates.

In Petionville, next to the capital, people used sledgehammers and bare hands to dig through a collapsed commercial center, tossing aside mattresses and office supplies. More than a dozen cars were entombed, including a U.N. truck.

Nearby, about 200 survivors, including many children, huddled in a theater parking lot, using sheets to rig makeshift tents and shield themselves from the sun.

About 3,000 police and international peacekeepers cleared debris, directed traffic and maintained security in the capital. But law enforcement was stretched thin even before the quake and would be ill-equipped to deal with major unrest.

An American aid worker was trapped for about 10 hours under the rubble of her mission house before she was rescued by her husband, who said to CBS’ “Early Show” that he drove 100 miles to Port-au-Prince to find her. Frank Thorp said he dug for more than an hour to free his wife, Jillian, and a co-worker, from under about a foot of concrete.

The international Red Cross said a third of the country’s 9 million people may need emergency aid, a burden that would test any nation and a crushing catastrophe for impoverished Haiti.

President Barack Obama promised an all-out rescue and humanitarian effort and American officials said they were responding with ships, helicopters, transport planes and a 2,000-member Marine unit, as well as civilian emergency teams from across the U.S.

“We have to be there for them in their hour of need,” Obama said.

Port-au-Prince’s ruined buildings fell on both the poor and the prominent: The body of Archbishop Joseph Serge Miot, 63, was found in the ruins of his office, according to the Rev. Pierre Le Beller at Miot’s order, the Saint Jacques Missionary Center in Landivisiau, France.    Senate President Kelly Bastien was among those inside the Parliament building, and a day later had stopped responding to rescuers’ cries, Latortue said.

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.

U.N. mission head Hedi Annabi of Tunisia was among about 150 people missing, mostly at the headquarters building, said peacekeeping chief Alain Le Roy. Brazilian Foreign Minister Celso Amorim said Annabi’s chief deputy, Luis Carlos da Costa, was missing as well.

Le Roy said only about 10 people had been pulled out, many of them badly injured.

Brazil’s army reported that at least 11 of its peacekeepers were killed. Jordan’s official news agency said three of its peacekeepers were dead.