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. "The lack of electricity is a problem, but technicians are trying to restore power".
There were power cuts and slight damage to the port of Zakynthos, near the epicentre.
The tremor was distinctly felt in Athens, some 300 km away, as well as across southern Italy, Albania and Malta.
From first reports, immeasurable damage has occurred at the Tower of the Castro Monastery in Strofades.
The natural disaster hit the western Greek tourist islands late Thursday evening. Two people were treated for minor injuries. He said engineers were checking all reports.
It struck at 1.54am local time and was followed about half an hour later by a 5.2 aftershock.
Greece's Geodynamic Institute has reported that the quake was of a magnitude of 6.4, and its epicenter was 33 kilometers. Measurements of quake strength can vary due to the equipment each institution uses and other factors. The village of Mouzaki is near the popular tourist resort of Laganas.
The majority of damages were reported on Zante, in particular at the largest port of the island, which nevertheless remained open.
The quake led to brief power cuts on Zakynthos.
Extensive damage was avoided because quake-prone Zakynthos had adopted seismic protection codes in construction, said Efthymios Lekkas, head of Greece's Earthquake Planning and Protection Organization said. More than 140 people were killed in an natural disaster north of Athens in 1999.
Picture shows the ground cracked in the harbour of Zante, after powerful natural disaster.