PORT-AU-PRINCE (Reuters) - A major earthquake rocked Haiti, killing possibly thousands of people as it toppled the presidential palace and hillside shanties alike and left the Caribbean nation appealing for international help.

A five-story U.N. headquarters building was also brought down by Tuesday's 7.0 magnitude quake, the most powerful to hit Haiti in more than 200 years according to the U.S. Geological Survey.

Reuters television footage from the capital, Port-au-Prince, showed scenes of chaos on the streets with people sobbing and appearing dazed amid the rubble. The presidential palace lay in ruins, its domes fallen on top of flattened walls.