An unidentified explosive device went off in the prestigious Mozhaisky Academy in St Petersburg, which trains officers for the army’s missile defence unit, according to acting governor Alexander Beglov.

Russia’s top investigative body said it has launched a criminal inquiry into conspiracy to murder.

Cadets evacuate the academyCadets evacuate the academy (Dmitri Lovetsky/AP)

Several windows were shut on the second floor of the building, and dozens of cadets were being led out of the gated compound.

A local news website, Fontanka, reported that the blast was caused by an improvised explosive device with an equivalent of 200 grams of TNT.