Haiti's envoy to the US said it was a "catastrophe of major proportions".
Buildings, including a hospital, are reported to have collapsed, with streets said to be blocked by rubble.
quake, which struck about 15km (10 miles) south-west of the capital,
was quickly followed by two strong aftershocks of 5.9 and 5.5
The tremor hit at 1653 (2153 GMT), the US Geological Survey said.
A Reuters reporter in Port-au-Prince said he had seen "dozens of dead and injured people" in the rubble of fallen buildings.
"Everything started shaking, people were screaming, houses started collapsing... it's total chaos," the reporter added.