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.
One woman on the southern island of Crete broke her leg while fleeing her home, and two men on another part of the island suffered minor cuts from shattered glass, police there said.
"The high state of alert has ended after it appeared there is no threat to loss of life. We're praying it stays that way," Public Order Minister Giorgos Voulgarakis said. "Rescuers (in Athens) are in helicopters, ready to go to anywhere. But it appears this will not be necessary ... The problems are being handled by local services."
"The alert is not over, but we have brought it down a level," he said.
On Kithira, police said the airport and 80 homes had been damaged and that a church was close to collapse in the village of Mitata. On Crete, a wall collapsed in a cafeteria in the port town of Hania without any injuries.
A church was also damaged in the village of Ayios Nikolaos in Lakonia, southern Greece while minor damage was reported on the southeastern Aegean island of Karpathos.
Phone services were also disrupted for at least half an hour on Kithira, Crete and parts of southern Greece.
The quake was felt across southern Italy — in particular around Naples, Catania and Sicily — but there were no reports of damage or injuries, officials said. Some people ran into the streets and others phoned authorities seeking information, but none reported damage, fire department and civil protection officials said.
The quake was also felt in Cairo, Egypt and in Amman, Jordan.
The Colorado-based USGS gave the quake's preliminary magnitude as 6.7. The reason for the discrepancy in magnitude was not immediately clear.
"I wish every earthquake was like this ... We were ready to deal with more serious damage. Luckily that didn't happen," Fire Chief Andreas Kois said.
In 1999, a 5.9-magnitude quake near Athens killed 143 people, injured about 2,000 and left thousands more homeless.