The US Geological Survey says the earthquake had a preliminary magnitude of 7.0 and was centered just 15 kilometers southwest of the capital, Port-au-Prince.
Experts describe it as the strongest earthquake in the area in more than 200 years.
"Many buildings have collapsed. Many people are looking for their relatives," Pastor Vasquez, spokesman for the Dominican embassy in Port-au-Prince, told the German Press Agency dpa.
"There is big, very big chaos. Sirens are heard, people are running desperately."
"I am shocked by the scale of the earthquake. Our sympathy and solidarity are with the victims of the disaster and their families," said German Foreign Minister Guido Westerwelle in Berlin after a telephone call with the German embassy in Haiti on Wednesday.
"The German government will assist the Republic of Haiti in every way possible."
Haiti forms half of the island of Hispaniola which it shares with the Dominican Republic. Most of the capital's 3 million inhabitants live in slums cobbled together from wood, tin and weak concrete.
Earthquake destroys capital
Witnesses managing to contact world media outlets say a number of government buildings, including the presidential palace, the headquarters of the UN peacekeeping mission and a hospital, were destroyed by the original quake or in the two aftershocks that registered 5.9 and 5.5 on the Richter scale.
Unconfirmed reports say many people are trapped under rubble and it is feared that hundreds have been killed. Haiti's ambassador to the US says the quake is 'a disaster of major proportions'
"A big problem is the lack of medical supplies. Even in the best hospitals in the country, the medicine is running out," said press spokesperson for the German humanitarian aid agency Diakonie Katastrophenhilfe, Rainer Lang, from Stuttgart on Wednesday. He is in email contact with the head of the organization's bureau in Port-au Prince, Astrid Nissen.
Haiti, one of the poorest countries in the world, has a population of around nine million.
Editor: Chuck Penfold