Police said the wounded man was out walking his rottweiler dog past 87 Bolshoi Prospect (Vasilyevsky Ostrov) when a red BMW 520 with Belarussian license plates drew up beside him and four other men.
All five dived for cover as the BMW's occupants opened fire with Kalashnikovs, hitting the alleged mafioso in the left thigh and the groin.
The man, identified by St Petersburg daily newspaper Smena as Mikhail Brav, is in hospital surrounded by private security personnel. A companion of his was hit nine times and died almost immediately, police said.
Smena reported that the wounded man had been accompanied by four bodyguards, three of whom were unhurt by the hail of gunshot.The other three fled the scene before police arrived.
"Two of them -- according to sources -- were members of the OMON [St Petersburg's paramilitary police]," Smena reported.
Police would not comment on the victims alleged Mafia links, though St Petersburg's Radio Baltika said its correspondents were on the scene within minutes of the attack and received convincing proof of the men's links to the 300-strong Tambovskaya gang.
Independent sources also confirmed that the wounded man was a leading member of the Tambovskaya mafia.
The assassins remain at large but police found the BMW within 2 kilometers (1.2 miles) of the shooting. It had been abandoned in the yard of a building on Vasilyevsky Ostrov's 23rd Line.
It contained two Kalashnikov AK-74, 5.45 mm assault rifles, one with an empty magazine and the other with one bullet left in it.
The Tambovskaya gang has been blamed for murder, extortion, kidnapping and theft.
A June report released by city police claimed the gang was stockpiling large amounts of cash, automatic weapons and hand grenades.
Local papers speculated that the attack was part of an internal power struggle. (SPP)