The US Geological Survey said the earthquake, which could be felt across central and northern Greece, was magnitude 6.3.
But the Institute of Geodynamics in Athens said earlier the quake had measured at a magnitude of 6.0.
According to the Athens observatory, the epicentre of the quake was 21 kilometres (13 miles) south of the town of Elassona, near Larissa and was eight kilometres deep.
There were at least three aftershocks following the main tremor -- including one at magnitude 4.0 -- and authorities warned there could be more.
Seismologist Gerassimos Papadopoulos warned of further "significant aftershocks", speaking on Skai radio.
However, experts stress that quake faults in the area rarely produce tremors larger than the one clocked on Wednesday.
The last major earthquake in the area was in the 18th century and was magnitude 6.2, Manolis Skordilis, a seismologist at Thessaloniki's Aristotle University, told state agency ANA.
Greece is located on a number of fault lines, and is sporadically hit by earthquakes.
But the quakes often happen at sea and do not often kill people or cause extensive damage.
The last fatal earthquake was in October, when a magnitude 7.0 hit in the Aegean Sea between the Greek island of Samos and the city of Izmir in western Turkey.
The majority of damage was in Turkey where 114 people were killed and more than 1,000 injured.
In Greece, two teenagers were reported dead on the island of Samos.