Police said they believed that a commercial dispute or blackmail attempt was behind the incidents.
Viktor Beltsov, a spokesman for the Emergency Situations Ministry, said 78 people sought medical care and 66 of them were taken to hospital, but none was assessed as being in a life-threatening condition.
Vyacheslav Stepchenko, a police spokesman, said the gas was preliminarily determined to be methyl mercaptan. The US Health and Human Services' Agency for Toxic Substances and Disease Registry says methyl mercaptan is a gas smelling like rotten cabbage that is manufactured for use in plastics and pesticides.
The agency's website said little is known about the gas's potential health effects. One person exposed to very high concentrations of the gas went into a coma and died, the site says.
Stepchenko said that a manager at a branch of the Maksidom home-supplies chain found a suspicious box before the shop opened and when she opened it, she found ampoules attached to wires and a timer. The woman inadvertently broke one of the ampoules and noticed a noxious smell, but apparently was not sickened, he said.
All those who sought medical care were from another branch of the chain, Stepchenko said. Boxes with glass containers attached to timers were found in two other stores by employees, who carried them outside and covered them with buckets; police explosives experts defused them, he said.
Officials of the store chain, which has outlets only in St Petersburg and sells furnishings, DIY material and other domestic articles, had told police that they had letters in recent weeks threatening to disrupt the holiday buying season, which in Russia is geared towards New Year’s Eve rather than Christmas Day.