The Athens Geodynamic Institute gave a preliminary magnitude of 6.5 and said the quake had an epicenter beneath the seabed 140 miles south-southwest of Athens. The U.S. Geological Survey initially put the preliminary magnitude at 7.3, but later revised it downward to 6.7. It struck shortly after noon.
Seismologists said aftershocks were possible, and Greek television stations warned people in areas near the epicenter to stay away from buildings.
"It was incredibly strong and lasted a long time," said Theodoros Salantis, mayor of Koroni, a seaside town near Kalamata, the closest city to the epicenter.
Seismologist Efthymios Lekkas said the quake struck at a depth of about 18 miles beneath the seabed, adding that initial indications were that it was the main earthquake rather than a preliminary temblor.
In January, a 6.5 quake struck another part of southern Greece. Its epicenter was deep underground and, while it had been felt across much of the country, it had not caused any injuries or serious damage.
Earthquakes are common in Greece, which is riddled with fault lines. In 1999, a 5.9-magnitude quake near Athens killed 143 people and left thousands more homeless
