The seven - six policemen and their driver - were killed early on Thursday while on patrol in Jharkhand's Palamau district, one of the many where Maoist rebels hold sway, a police spokesman said.
The explosion was caused by a landmine, police said, adding suspicion fell on Indian Maoist rebels who are said to be active in Palamau.
The district is among 24 seats in the 81-member Jharkhand legislature going to the polls in the first phase on Thursday.
Polling has been divided into three phases in Jharkhand and neighbouring Bihar for security reasons.
Both states are affected by a Maoist insurgency and election officials said about 100,000 security personnel were on duty to ensure a smooth ballot.
Calls for boycott
The rebels last week called for a boycott of the polls, saying elections were no solution to the problems of the rural poor whom they say they represent.
Thousands have died since the 1960s in rural India in uprisings by Maoist fighters. In the Bihar district of Kaimur, police said an overnight patrol team recovered 10kg of dynamite.
Thousands have died in India in
uprisings by Maoist fighters
Bihar, where caste wars are frequent, has millions of unemployed youth and is considered to be India's poorest and most lawless state.
Also going to the polls on Thursday was the northern state of Haryana, neighbouring the Indian capital New Delhi.
Television footage showed people queuing up at polling stations across the state where voting for all 90 legislature seats were being held on Thursday, a polling official said.