This is an Eastern Orthodox cathedral in Sofia, the capital of Bulgaria.
Built in Neo-Byzantine style, it serves as the cathedral church of the Patriarch of Bulgaria.
It is one of the largest Eastern Orthodox cathedrals in the world and is one of Sofia's symbols and primary tourist attractions.
It occupies an area of 3,170 sq m. (34,100 sq ft) and can hold 10,000 people inside.
It is the second-largest cathedral located on the Balkan Peninsula, after the Cathedral of Saint Sava in Belgrade.
Reference: Wikipedia