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The tremblor was recorded west of Athens at 6.02 am (0402 GMT) in the Corinthian Gulf, with assessments giving it a focal depth of 5 kilometres, the National Observatory of Athens said on its website.
"It felt like two earthquakes in a row! It was quite strong. It woke me up," one person wrote on the European-Mediterranean Seismological Centre website.
The US Geological Survey said the epicentre was 79 kilometers west-north-west of Athens.