In Turkey, about about 270 people were hospitalized, most with minor injuries, as a tremor shook buildings, the governmentdiaster and emergency department (AFAD) said.
The quake also rattled Turkey's most populous city, Istanbul, as well as the Aegean coastal city of Izmir and the popular tourism province of Antalya on the Mediterranean coast.
One person was in critical condition after jumping from a balcony in the western Turkish town of Canakkale, AFAD said. Hurriyet Daily News reported 30 people injured jumping out of apartments in the town and patients in one hospital were evacuated after cracks emerged in the building.
The U.S. Geological Survey (USGS) said the quake occurred 69 km (42 miles) south-southwest of the Greek city of Alexandroupolis, between the islands of Lemnos and Samothrace, at a depth of 10 km (6 miles).
"It lasted very long and it was very intense. We haven't got the full picture of the damage caused yet," the mayor of Lemnos, Antonis Chatzidiamantis, told Mega TV.
One woman British tourist was slightly injured at the Aegean island's airport when part of the ceiling collapsed, Chatzidiamantis said.
"It was very strong - cupboards, glasses, coffee cups, they all broke," an elderly Lemnos resident told Greek radio.
Greek police said the quake had caused minor damage to shops and houses on the two islands.
Seismologists described the quake, which was felt across Greece, as "severe" and warned that aftershocks measuring over 5.0 magnitude were likely. "It will certainly have a very rich aftershock activity," seismologist Costas Papazachos told Ant1 TV. "There is obviously some reason for concern...we could easily have aftershocks of 5, 5.5 or 6 magnitude," he said.
Greece is often buffeted by earthquakes. Most cause no serious damage but a 5.9 magnitude quake in 1999 killed 143 people. In Turkey, more than 600 people died in October 2011 in the eastern province of Van after a quake of 7.2 magnitude and powerful aftershocks. In 1999, two massive earthquakes killed about 20,000 people in Turkey's densely populated northwest.
Saturday's tremor was also felt throughout Bulgaria, where two women in the southwestern cities of Smolian and Petrich collapsed under stress, the government said in a statement.
"The whole house was shaking. It was scary," a resident in Bulgaria's second biggest city Plovdiv, Gergana Petrova, told Focus News agency.
(Reporting by Karolina Tagaris and Renee Maltezou in Athens, Asli Kandemir and Nick Tattersall in Istanbul, Jonny Hogg in Ankara and Tsvetelia Tsolova in Sofia)