Suspected Maoist rebels set off two bombs in Chhattisgarh on Saturday, killing 13 people, most of them paramilitary soldiers and officials charged with holding elections in the region.
The attacks, half an hour apart, were the most serious since voting to elect a new central government began last week in a six-week process to allow security forces to move across the country.
The first explosion took place in a bus in Bijapur carrying election officials who were on their way back after completing the vote. Seven people were killed.
A second bomb hit an ambulance in the thickly forested Bastar region killing five members of the Central Reserve Police Force and their driver, said RKVij, the head of anti-Maoist operations.
It was not clear why the soldiers were travelling in the ambulance, but in the past government officials are known to be have used such vehicles to avoid attacks by the Maoists.
The rebels have operated for decades across a wide swathe of central and eastern India, and grew in strength during recent times in areas where poor, tribal villagers came into conflict with mining companies seeking resources for industrialisation.
The Maoists seek the violent overthrow of the Indian state, accusing it initially of taking over land from poor peasants and now plundering the mineral wealth of states likes Chhattisgarh.