In Istanbul and in other parts of Turkey residents fled homes in panic after the quake struck at midday.
The private Dogan news agency said the temblor caused damage to some old houses on the island of Gokceada, off Turkey's northern Aegean coast, and 30 people were taken to a hospital with minor injuries.
The quake struck at 1455 IST (0925 GMT) southwest of the Greek island of Samothraki, 210 kilometres east of Thessaloniki and 296 kilometres northeast of the capital Athens. It was also close to the Turkish island of Gokceada and the Greek island of Lemnos.
The Institute of Geophysics at the Aristotle University of Thessaloniki said the temblor announced a magnitude of 6.3, the US Geological Survey initially reported a magnitude of 6.4, later revised to 6.9.
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There were divergences as to the depth, as well.
The USGS reported a depth of 10 kilometres but the Athens Geodynamics Institute has reported 27 kilometres.
The temblor was widely felt, including in Thessaloniki, the west coast of Turkey and as far away as Bulgaria and Istanbul. Several strong aftershocks, of yet undetermined magnitude, have taken place.
"We are currently analysing the aftershocks and are on alert," he added.