The Emergency Situations Ministry earlier said a blast hit the sprawling Cherkizovsky clothes market at about 10:30 a.m. Moscow time (6:30 a.m. GMT) and destroyed about 200 square meters of roofing.
Vladimir Resin said: "A bomb went off, killing 10 people, including two children, and 31 people have been taken to hospital."
However, Prosecutor Yury Syomin stuck to earlier figures that eight people had been killed. He added that another 41 had been hospitalized.
A police representative also said only one blast had ripped through the market, correcting earlier reports that a device had gone off first before a gas canister exploded.
"An improvised explosive device detonated at the market," police said. "One explosion took place."
Investigators said the power of the explosion was equivalent to 200 to 500 grams of TNT.
The Prosecutor General's Office said it was investigating the incident as a premeditated murder of two or more people.
Over 30 rescuers are working at the scene, and senior police officials and prosecutors have arrived there.