A new earthquake hit central Greece near the city of Larissa on Thursday, the same region where an even stronger tremor left 11 people injured the day before, the Athens seismological observatory said.
The quake, which the US Geological Survey said was of 5.6 magnitude, caused rock falls close to the UNESCO World Heritage-listed Meteora orthodox monasteries in Kalabaka, emergency services said.
No injuries were immediately reported from the new quake, which was felt 20 kilometres (12 miles) from Elassona, near Larissa, the observatory said.
But rescue services said there were rock falls and new damage to buildings already weakened by the previous day's quake.
And the state television channel ERT showed footage of panicked residents on the streets.
The Athens observatory described the tremor as "very strong" and said it occurred at 8:38 pm (1838 GMT) some 16 kilometres from Elassona and 244 kilometres north of Athens.
The head of Greece's quake protection authority, Efthymis Lekkas, told Skai radio that the new tremor was around five kilometres from the epicentre of the quake on Wednesday.
"This new earthquake activated a second fault located near the one on Wednesday, which is rare in Greece," said Thanassis Ganas, a seismologist at the Athens observatory.
"Normally, when there is a main earthquake like the one on Wednesday there are only aftershocks. It is rare to have a second main earthquake in such a short period," he also told Skai radio.
Greece is located on a number of fault lines, and is sporadically hit by earthquakes. But they mostly occur at sea and do not cause casualties.
In October, a 7.0 magnitude quake struck in the Aegean Sea between the Greek island of Samos and the city of Izmir in western Turkey. Two teenagers died on the island of Samos in a building collapse.