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The quake struck at 16:29 pm (13:29 GMT) near the Aegean island of Karpathos, an institute seismologist told AFP.
The epicentre was some 360 kilometres (224 miles) southeast of Athens and 20 kilometres west of Karpathos, the institute said.
Authorities on Karpathos and the neighbouring of Rhodes and Crete, where the tremor was also strongly felt, said no damage or injuries had been reported.
Greece accounts for half the earthquakes on the European continent.