Firefighters stand outside a supermarket after a blast in Saint Petersburg. Pic/AFP

At least 10 people were injured in a blast inside a shop in Russia's second largest city, St. Petersburg, the media reported. According to emergency services sources, an unidentified device exploded in a shop, which prompted the evacuation of at least 50 people, Sputnik news agency reported. Preliminary data shows that an explosive device found at the scene contained 200 grams of TNT, CNN reported citing RIA news agency.