A woman named Shanti Devi was injured when she picked up from the ground the crude bomb mistaking it for an interesting object and it exploded in her hand. Her husband and a neighbour's boy standing nearby were also wounded.
All the three injured were admitted to hospital.
Seven constituencies are going to polls on Wednesday in the fourth of the six-phase Lok Sabha elections in Bihar to decide the fate of 94 candidates.
The seats are Madhubani, Jhanjharpur, Madhepura, Darbhanga, Samastipur (SC, Reserved), Begusarai and Khagaria, all in the Kosi river belt.