The blasts began yesterday night and continued for hours after being sparked by efforts to dispose of munitions at an arms depot close to the town of Chapayevsk in the Samara region around 1,000 kilometres from Moscow.
The depot contained around 18 million shells, the regional Investigative Committee said in a statement today.
Russian television showed footage of huge columns of black smoke rising from the depot, while a correspondent for Rossiya 24 television at the scene said that explosions rang out every minute overnight.
A total of 48 people needed medical treatment and 11 were hospitalised, the regional Investigative Committee said, without specifying the nature of their injuries.
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The emergency situations ministry used aircraft to drop water on the fire.
The situation stabilised yesterday morning, the ministry said, with explosions ceasing around 6 am, while a fire continued to blaze.
Samara regional police said yesterday evening that the explosions began during work aimed at disposing of the old shells.
Such accidents occur frequently at Russian arms depots that store vast stockpiles of old munitions in primitive conditions with little regard to public safety, and often close to settlements.
In October 2012, a depot containing 4,000 tonnes of munitions exploded in the Urals region, causing thousands to be evacuated .
In May 2012, a similar accident caused the death of six soldiers in the Nizhny Novgorod region of central Russia.