Russian media said the injured were being treated for concussion, burns and shrapnel wounds.
Russia's Interfax news agency, citing an unnamed St Petersburg police official, said investigators were considering "hooliganism" as the main theory behind the blast.
Anti-Moscow fighters linked to Russia's Chechnya region have mounted a spate of bomb attacks on civilians in Russia but there has been no major attack outside the turbulent North Caucasus region for more than two years.
McDonald's restaurants in Russia have been targets of both gang
disputes and terrorist attacks in the past.
In 2002, a car bombing at a McDonald's restaurant in Moscow killed one
person and injured eight.
Several Chechen men were later convicted in the blast, which prosecutors said was part of a series of attacks planned by Chechen rebels.
In 1998, a small bomb exploded at an unfinished McDonald's restaurant in
another part of St Petersburg.