Before the October Revolution of 1917, there were over 100 orthodox cathedrals and churches in St. Petersburg.
After the Revolution, the number was reduced for various reasons: some were blown up and destroyed, others were used as cargo houses for enterprises. 
   Some churches, the more fortunate ones, were turned into museums. For example, the Kazan Cathedral was formerly the Atheism and Religion Museum, and the St. Isaac Cathedral was one of the city museums. This allowed  their interior decorations to be well preserved..
   There were only a few active churches in St. Petersburg (then Leningrad) by the end of 80s. After the beginning of Perestroika, the process of the revival of churches had started;  they were returned to the Petersburg Diocese and renovations and reconstructions began. Right now there are about 40 active churches, and more appear every year...

Orthodox Churches

Non-Christian Churches

Christian Churches