Repair people found 1.23 kg of hexogen, 19 detonators and 18 fuses in an uninhabited house on Thursday.
The bag with explosives was found at 20 Leningradsky Prospekt during repairs in the attic of the building, an anonymous source told RIA Novosti.
The explosives had been in the house since the early 1990s it was reported.
A day earlier it was reported that a bag with two TNT blocks of 300 g each was found at the same address, but it is unclear whether there were two separate incidents.
Until recently, the building had been used as headquarters of Bulgarian cultural institute, Friends of Bulgaria union and a Bulgarian cultural center.
The three-story building was set to be demolished.