A strong earthquake of magnitude 6.8 has struck off Greece, the US Geological Survey says, but there were no initial reports of damage or casualties.
The epicentre of the quake, initially reported as a magnitude 7.0, was in the Ionian Sea, some 130km southwest of Patras in the Peloponnese.
A fire brigade official in Athens said there were no early reports of damage or casualties.
Attention was focused on the island of Zakynthos, where reports indicated power was out.
"It was strong, things fell of the shelves," a witness in Katastari, Greece, 36km north of the epicentre, wrote on the European-Mediterranean Seismological Centre (EMSC) website.
Local news websites reported that a 15th century monastery in the Strofades area of the island had been damaged.
The tremor was felt in Greece's capital, Athens, and as far away as Italy.
Italy's Il Messaggero online news site reported that the quake had been felt hundreds of kilometres away in southern Italy. Firefighters in Sicily, Calabria and Puglia received thousands of telephone calls for information about the quake.
The quake was fairly shallow, only 16.5 km below the seabed, which would have amplified shaking.
It struck at 1.54am local time and was followed about half an hour later by a 5.2 aftershock.
Greece straddles two tectonic plates and often gets earthquakes.
While the US Geological Survey put the quake at 6.8, Greece's Geodynamic Institute reported a tremor of 6.4, 50km south of Zakynthos.