The Athens Geodynamic Institute said the quake, with a preliminary magnitude of 6.9, occurred off the coast of the small island of Kithira, about 125 miles south of Athens at 11:34 p.m.
Its epicenter was at a depth of about 43 miles beneath the sea — which likely contributed to the lack of major damage or serious injuries, seismologists said. It shook the entire region, from Italy in the west to Egypt and Jordan in the east, and was felt throughout Greece.
Clarice Nassif Ransom, a Washington spokeswoman for the U.S. Geological Survey, said scientists project that as many as six million people may have felt the earthquake.
"It was a very powerful quake which shook all of Greece," said Giorgos Stavrakakis, head of the Athens' Geodynamic Institute. "There have been dozens of aftershocks, four with a magnitude of 5 ... The quake occurred deep undersea and that's what saved us."
No tsunami warnings were issued from the quake, which lasted about 30 seconds. Aldo Winkler, a researcher at Italy's Institute of Geophysics and Vulcanology, said a tsunami was virtually impossible and damage in Italy unlikely because the magnitude of the quake was not high enough and its epicenter was too deep under the sea.
Stavrakakis said the region "regularly gives out strong quakes, so we are not surprised. But we don't expect any serious aftershocks."
The Greek government initially put the armed forces and emergency services on high alert, but later reduced the level, saying damage was localized and only three minor injuries were reported.