Five tremors measuring around 5.0 points on the open-ended Richter scale shook Lefkas throughout Thursday after a 6.4-point quake struck at 8:15 am (0515 GMT).

The quakes caused landslides which blocked roads, collapsed 30 uninhabited houses and seriously damaged around 50 inhabited ones, according to Greek officials.

"Greek tourists have all left. The foreigners stay back, maybe because they can't leave by car," Vasso Ligda, who rents rooms at the southern Lefkas port village of Vassiliki, told Agence France Presse.

Falling rocks

A Czech tourist who fell off his bike during the morning quake is on a life support machine in a hospital at the western Greek city of Ioannina.

Two Britons were hit by rocks during the same tremor and were being treated in the same hospital. The Britons suffered leg and head injuries but their lives were not in danger, hospital officials said.

Another 60 people were slightly injured, a Greek health ministry official said late Thursday.

Landslides blocked roads

The Greek civil protection service said around 180 tents had been sent to Lefkas.

According to the Greek deputy minister for public works and environment Ioannis Tsaklidis, up to 50 houses have been temporarily declared unfit for housing throughout the island.

Authorities asked their inhabitants to spend the night elsewhere.

"My house is okay as are all others in Vassiliki, but I'll spend the night outside. We have no tents here but we don't need them since it's summer," Ligda said.

"No tourists have left, but me and my husband will stay up on alert all night," said Eleni Stavraka, who runs a hotel with her husband in the Lefkas resort of Nydri.

Underwater epicentre

The morning quake was felt throughout the western Greek mainland, as well as on the busy tourist island of Corfu, north of Lefkas, but no injuries or serious damage were reported there.

Archaeological monuments in the Lefkas area suffered damage, Greece's culture ministry said in a statement.

Greek geologists cautiously predicted the string of afterquakes probably meant the 6.4-point tremor was the biggest.

"The seismic activity is normal," said Gerassimos Papadopoulos, head of the Athens Geodynamic Institute.

The earthquake caused minor damage, compared with its size, because its epicentre was under the sea, they added.

Western Greece is a highly earthquake-prone area. Sunday will mark the 50th anniversary of a destructive earthquake that flattened houses on the island of Cephallonia to the south of Lefkas.