A major earthquake rocked Haiti on Tuesday, crushing thousands of buildings, including the presidential palace and the UN peacekeeping headquarters, and trapping untold numbers of people in the rubble of the capital city.

The devastation from the magnitude-7.3 quake, the strongest ever recorded on the poor Caribbean island, was so complete that it seemed likely the death toll would run into the thousands, according to reports monitored here.

The offices of the finance and public works ministries, the parliament building, a hospital and a cathedral were also among the many structures that were severely damaged or destroyed in Port-au-Prince, a city of about 2 million people.

Much of the presidential palace pancaked on itself but President Rene Preval and his wife survived the earthquake.

Preval told the Miami Herald that the toll from the quake was "unimaginable" and estimated that thousands had died.

"Parliament has collapsed. 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," the president told the newspaper.

The earthquake struck at about 4:53 PM local time (2153 GMT) ,which was centered 15 km southwest of the capital city at a depth of 10 km, the US Geological Survey (USGS) said.

The USGS said the earthquake was followed by a tsunami seven minutes later and two aftershocks of 5.9-and 5.5-degrees respectively.

Power supplies were cut off in affected areas and communications were scattered. Heavy casualties were feared although no official reports were yet available.

According to the International Federation of the Red Cross, as many as 3 million people may have been affected by the massive earthquake.

Red Cross spokesperson Paul Conneally said that at least one or two days more would be needed to determine a much clearer picture of the destruction.

"There are massive, massive, massive challenges," Conneally said.