The U.S. Geological Survey (USGS) put the preliminary magnitude at 6.8 with a depth of just 16 kilometers (10 miles). "The lack of electricity is a problem, but technicians are trying to restore power".
Attention was focused on the island of Zakynthos, where reports indicated power was out.
The tremor was felt in Athens, the Greek capital, and as far away as Albania, Italy, Libya and Malta.
The quake rattled the whole of western Greece and was felt in Kefalonia, Corfu, and Crete according to the USGS.
On one of the nearby Strofades islands, an ancient monastery was badly damaged after a large part of a tower collapsed.
Residents also left their homes in the city of Patras on the Greek west coast, reports said.
Greece's main natural disaster monitoring centre, Athens' Geodynamic Institute, measured the magnitude at 6.4, and said it had a depth of three miles.
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It struck at 1:54am (2254 GMT Thursday) and was followed about half an hour later by a 5.2 aftershock.
Damages in the port infrastructures, to old houses and minor damages to stores and houses all over the island were reported.
The Ionian island of Zakynthos is one of Greece's most popular tourist destinations, along with Crete, Kerkyra and Paxi, according to the country's tourism minister Elena Kountoura.
"Zakynthos was tested by today's natural disaster".
The most recent quake to claim lives, was in 1999 where the magnitude was recorded at 5.9 which killed 143 people on the outskirts of Athens.
Zakynthos has had severe earthquakes in the past, and as a result has a very strict building safety code.
Picture shows the ground cracked in the harbour of Zante, after powerful quake.