The Festival and Congress Center in the Black Sea city of Varna has received a bomb threat, prompting its closure and evacuation of visitors and employees.

The call has been placed slightly after 6:30 pm Tuesday on the emergency 112 hotline with a male voice alerting about homemade explosives inside the building.

Police and a police bomb squad have searched the premises, but did not find any explosives. The probe has established that the call has been placed from a phone booth on the city's main pedestrian street, but the "prankster" remains unknown and at large.

An increased number of bomb threats for shopping centers, courts and schools in Sofia and several other major Bulgarian cities was registered in the last few months. On Monday, two schools in the capital were closed and students evacuated after receiving bomb threats at the very first school day for 2013.

No traces of explosives have been found in any of these cases.