* Tremor felt in neighbouring Turkey, Bulgaria
* Panicky residents rush into streets
* No immediate reports of serious damage, injuries
* Seismologists expect many aftershocks (Adds quotes, damage, injuries)
ATHENS/ISTANBUL, May 24 (Reuters) - An earthquake of 6.4 magnitude struck off the coast of northern Greece on Saturday and was felt as far away as neighbouring Turkey and Bulgaria but there were no reports of serious casualties or destruction, police and fire brigade officials said.
The U.S. Geological Survey (USGS) said the quake occurred some 77 km (48 miles) south-southwest of Alexandroupolis, between the islands of Lemnos and Samothrace, at a depth of 10 km (6 miles).
Greek police said the quake had caused minor damage to shops and houses on the two islands.
The USGS downwardly revised its first reading of 7.2.
Greece, at the southeastern end of Europe, is often buffeted by earthquakes. Most cause no serious damage but a 5.9 magnitude quake in 1999 killed 143 people.
"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.
He said one woman - who a police official said was a British tourist - was slightly injured at the Aegean island's airport when part of the ceiling collapsed.
"It was very strong - cupboards, glasses, coffee cups, they all broke," an elderly Lemnos resident told Greek radio.
The Athens Geodynamic Institute put the quake at 6.3.
Across the Aegean in parts of western Turkey on Saturday morning, panicked residents rushed into the streets as a tremor shook buildings, local media reported.
The quake also rattled Turkey's most populous city, Istanbul, as well as the Aegean coastal city of Izmir, according to Reuters witnesses, but there were no immediate reports of damage.
Hurryiet Daily News said that the quake was also felt in the popular tourism province of Antalya. Thirty people were reported injured after jumping out of their apartment windows in the western town Canakkale, according to the newspaper.
Earthquakes are a frequent occurrence in Turkey, which is crisscrossed by geological fault lines. In October 2011, more than 600 people died 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 caused panic in the southern Bulgarian city of Haskovo, national radio reported, but the country's Civil Protection agency said there was no reports of casualties.
Greek 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. (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; Editing by Mark Heinrich and Stephen Powell)