A 5.6 magnitude earthquake shook Bulgaria's capital Sofia early on Tuesday, causing residents to rush into the streets, the civil defence office said.
The quake, which occurred at 3 a.m. local time, shook apartment buildings and rattled windows but caused no casualties or damage, Nikolay Nikolov, an official from the office was quoted as saying by the national radio, citing initial reports.
The U.S. Geological Survey said the temblor was centred about 24 kilometres west of Sofia and occurred at a relatively shallow depth of 9.4 km.
USGS initially reported the magnitude as 5.8.
The civil defence office said the quake's epicentre was near the western town of Pernik.
Many people in the Bulgarian capital fled their homes and gathered in the streets.
"I've never experienced anything like this," Ivanka Georgieva, who lives on the eighth floor in a residential block, told Reuters. "It was very strong and it was frightening."
The quake was felt across the southwestern part of Bulgaria.
The small Black Sea country of fewer than 8 million people lies in Europe's earthquake-prone Balkan region.