Greece’s Geodynamic Institute reported the magnitude as 6.4, and put the quake 50 km (31 miles) south of the island of Zakynthos. That compares with a figure of 6.8 assessed by both the US Geological Survey or USGS and the European-Mediterranean Seismological Centre (EMSC).
The tremor was felt in Athens, the Greek capital, and as far away as Albania, Italy, Libya and Malta.
The epicentre was in the Ionian Sea, about 130 km (81 miles) southwest of Patras in the Peloponnese, the USGS said.
It was felt particularly on Zakynthos, also known as Zante, which was rattled by a series of strong aftershocks, the highest at 5.6, and where power was briefly disrupted.
Three people were taken to hospital, two of them slightly injured, a spokesman for Greece’s civil protection agency told Reuters.
News websites said a 15th-century monastery had been damaged, with one, Zante News Time, displaying images of masonry that had fallen from a building facade and items dislodged from shelves in shops.
The power cut and slight damage to the port of Zakynthos, near the epicentre would not affect port operations, officials said.
“We are not facing any particular problems,” Zakynthos Mayor Pavlos Kolokotsas told Greek state broadcaster ERT. “Calm is being restored.”
Italy’s Il Messaggero online news site said the quake had been felt hundreds of kilometres away in southern Italy. Firefighters in Calabria, Puglia and Sicily received thousands of telephone calls seeking information about the quake.
“The energy unleashed, based on the angle of the faultline, fanned out towards Italy,” said Efthymios Lekkas, head of Greece’s Earthquake Planning and Protection Organisation.
Extensive damage was avoided because quake-prone Zakynthos had adopted seismic protection codes in construction, he added.
An official confirmed that a tsunami alert had been lifted. The EMSC European quake agency said sea levels had risen slightly, by about 20 cm (7.87 inches), but the increase could be higher locally.
The quake was fairly shallow, according to the USGS, just 14 km (8.7 miles) below the seabed, which would have amplified shaking.
It struck at 1:54am (2254 GMT). Greece straddles two tectonic plates and often suffers earthquakes.