At least 42 people have been injured, 14 hospitalized, and two remain missing.
The regional health ministry said 19 people were treated for injuries after the explosion; none of the injuries were considered severe.
A roaring blast from the explosion in the central city of Dzerzhinsk was so loud it was heard in nearby towns and cities, including Nizhniy, which is around 19 miles away, Russian media reported.
Five of the factory buildings were destroyed and residents were asked to keep their windows closed.
Earlier health ministry officials told news agencies that the wounded had "shrapnel wounds of mild and moderate severity" and that they were receiving medical assistance.
According to a source quoted by Interfax news agency, the factory was used to produce and store high-explosive bombs.
V Gorode N said two months ago there was another explosion at the plant, but at the time no injuries were reported.
Local fire fighters told agencies they had no information about any more people still inside the plant, which was set up in 1953.
Representatives of Kristall told Interfax that five people were working in the area where the explosion occurred and were safely evacuated.
According to the Ministry for Emergency Situations, an 800-square-metre area at the factory site had been on fire, and the fire had also spread to a nearby forest.
Russian state TV Rossiya 24 showed footage of a plume of smoke rising over the explosives factory.