A light earthquake was registered in Bulgaria around midnight, the Seismological Centre at the Bulgarian Academy of Science reported.
The quake measured 2.8 on the Richter scale and its epicenter was located 180km southeast of Sofia.
There were no immediate reports of damages, the State Agency for Civil Protection informed.
Meanwhile a 5.6 earthquake struck off western Greece in the Ionean Sea early on Monday. The epicentre of the quake was 340km west of Athens in the sea, the officials said.
There were no reports of damage or injuries.