The explosions occurred at a park in Utsunomiya, some 100 kilometres north of Tokyo, shortly after 11:30 am local time, a fire department spokesman said. It was not immediately clear what caused the blasts.
"One person was found dead," the fire department spokesman told AFP, without elaborating further.
Public broadcaster NHK said a body badly damaged in the blast was found at a bicycle parking space for the park.
"The sex and ages of the three are not known yet," another local fire department official said, adding one blast hit a car parking lot for the park.
A man told NHK that he "smelled gunpowder in the area" after the explosions.
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