Those killed in the explosion Tuesday night in Idinthakarai, a fishing village not far from the Kudankulam Nuclear Power Plant in Tamil Nadu state, included two children, NDTV and other media reports said.
The report said an anti-nuclear activist had been arrested.
Officials at the plant, built with Russian help, said the facility was not affected by the explosion and was operating normally.
Police said the explosion occurred in a home in the village where the explosive devices were being assembled and two live bombs were recovered at the scene. The explosion damaged two more homes.
NDTV quoted police as saying they suspect some of those who died in the blast were activists of the People's Movement Against Nuclear Energy, which has spearheaded the movement against the Kudankulam nuclear plant.
There had been massive protests in the past two years in the area opposing the construction of the plant.
The Press Trust of India said police had also booked some anti-nuclear activists.
Police said the bomb may have gone off accidentally.
The Hindu newspaper said an undetermined number of people were injured in the blast, some seriously. The report said authorities rescued four people, including two children trapped under the debris.