"There are a lot of schools that have a lot of dead people in them."
Mr Preval later said that Hedi Annabi, the Tunisian head of the UN stabilisation mission in Haiti (Minustah), had died after the UN HQ building was destroyed.
The UN said it could not confirm the news but that Mr Annabi had been in the building at the time and was likely to be under the rubble, along with many others.
The main prison in Port-au-Prince has also collapsed, with a UN humanitarian spokeswoman saying there had been reports of escaped inmates.
A number of nations, including the US, UK and Venezuela, are gearing up to send aid.
Speaking in Washington, US President Barack Obama vowed "unwavering support" for Haiti after what he called a "cruel and incomprehensible" disaster.
He said he had ordered "a swift, co-ordinated and aggressive effort to save lives" and that the first US rescue teams would arrive later on Wednesday.
US Secretary of State Hillary Clinton announced that she was cutting short a trip to the Pacific in order to return to Washington because of the earthquake.
A US Navy aircraft carrier is expected to reach Haiti in a couple of days and a number of smaller vessels are already in the area, US defence officials said.
Rajiv Shah, of the US Agency for International Development (USAID), said US teams were on their way to Haiti with specialised rescue equipment and that some efforts were already under way on the ground.
The quake, which struck about 15km (10 miles) south-west of Port-au-Prince, was quickly followed by two aftershocks of 5.9 and 5.5 magnitude.
The first tremor had hit at 1653 local time (2153 GMT) on Tuesday, the US Geological Survey said. Phone lines to the country failed shortly afterwards.
Ban Ki-moon: 'We are facing a major humanitarian emergency'
UN officials said at least 14 people had died when the UN's five-storey headquarters and two smaller buildings in Port-au-Prince collapsed. Around 100 were still thought to be missing, many feared to be under the rubble.
Ten Brazilians, three Jordanians and one Haitian had been confirmed killed, a senior UN official said, adding that the number was likely to rise.
Stressing a major international relief effort would be needed, UN Secretary General Ban Ki-Moon said the UN would immediately release $10m (£6.15m) from its emergency response fund.
He said aerial reconnaissance showed Port-au-Prince had been "devastated" by the quake, although other areas were largely unaffected.
The airport in Port-au-Prince is operational despite the collapse of the control tower, the UN said, allowing aid to start arriving soon.
The Brazilian army, which has the biggest UN contingent in Haiti, has said a large number of its peacekeepers are missing.
China has indicated in reports in state media that eight of its peacekeepers are dead, with another 10 unaccounted for.
Objects were falling from shelves, there was debris crashing all around. I clung on to the babies and shielded them as best I could
A spokesman for medical charity Medecins sans Frontieres (MSF) said it was able only to offer basic care to the "massive influx" of survivors seeking help because all its buildings had been destroyed.
"Unfortunately what we are seeing is a large number of patients in critical condition," he said.
There were some reports of looting overnight.
Haiti is the poorest country in the western hemisphere and has suffered a number of recent disasters, including four hurricanes and storms in 2008 that killed hundreds.
With communications destroyed by the earthquake, it is not yet possible to confirm the extent of the destruction, although there were reports on Wednesday of many bodies piled in the streets.
People in the capital were lifting sheets on bodies to try to identify loved ones.
Gary Duffy, BBC News, Sao Paulo
Given Brazil's central role in leading the military side of the UN peacekeeping mission in Haiti the earthquake has caused a lot of concern and shock here.
The authorities here say 11 soldiers have died and there have been several injuries, and there are fears this number could rise.
It has also been confirmed that Zilda Arns, a prominent Brazilian aid worker and paediatrician, has been killed. She was a sister of the retired Cardinal Paulo Arns, a major figure in the Catholic Church here.
Some soldiers have managed to make contact to reassure relatives, but given the damage to infrastructure at Brazilian bases, communication is proving difficult, even for the government.
Damage has also been reported in the towns of Jacmel and Carrefour, near Port-au-Prince.
Guido Cornale, a representative of the UN children's agency Unicef, in Jacmel, said it estimated more than a fifth of buildings there had been destroyed.
The Red Cross is dispatching a relief team from Geneva and the UN's World Food Programme is flying in two planes with emergency food aid.
The Inter-American Development Bank said it was immediately approving a $200,000 grant for emergency aid.
The head of the International Monetary Fund (IMF) said it would co-ordinate with other international agencies to offer help as swiftly as possible.
The World Bank also said it was sending a team to assess the damage and plan recovery. It said its offices in Port-au-Prince had been destroyed but that most staff were accounted for.
The UK said it was mobilising help and was "ready to provide whatever humanitarian assistance may be required".
HAITI COUNTRY PROFILE
Half of Caribbean island of Hispaniola
History of violence, instability and dictatorship
Population of 10 million people
Most live on less than $2 a day
Democratic rule restored in 2006
Economy in ruins and unemployment is chronic
UN peacekeepers deployed - foreign aid seen as vital
Massive deforestation has left just 2% forest
Storms and hurricanes in 2008 left almost 800 dead
Canada, Australia, France and a number of Latin American nations have also said they are mobilising their aid response.
Pope Benedict XVI has called for a generous response to the "tragic situation" in Haiti.
Emmet Murphy, who works for a non-governmental organisation in Haiti, told the BBC: "I was driving through the mountains on my way home to Jacmel when the car started to shake. It was like a very strong wind was blowing and I nearly lost control of the car.
"I drove further and found the road totally blocked by a massive landslide on the road. I just knew that if I had reached that spot five minutes earlier, I would have been killed."
Blogger Troy Livesay, in Port-au-Prince, wrote: "Thousands of people are currently trapped. To guess at a number would be like guessing at raindrops in the ocean. Precious lives hang in the balance.
"When pulled from the rubble there is no place to take them for care. I cannot imagine what the next few weeks and months will be like."
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