The victims were killed by a landmine after escorting election staff to Palamau district, one of the many areas where Maoist rebels hold sway, senior police official M.P. Rao told the NDTV news channel.
"There is no doubt about it. It is violence unleashed by the Maoists," Rao said.
In the same area, four Maoists were killed when they tried to disrupt polling at Dandila village in Palamu district, police said. Their bodies were taken away by rebel colleagues.
Palamau, known to be a hotbed of insurgency, is among 24 seats in the 81-member Jharkhand legislature going to the polls in the first phase Thursday.
Polling has been divided into three phases in Jharkhand and neighbouring Bihar for security reasons.
Both states are affected by Maoist insurgency and poll officials said about 100,000 security personnel were on duty to ensure a smooth ballot.
But two policemen and a woman were killed by Maoists outside a polling station in Bihar's Gaya district, 125 kilometres (78 miles) south of state capital Patna.
Elsewhere, two women were killed in the cross-fire between two rival political groups in Sasaram district, 200 kilometres (124 miles) Southwest of Patna, police said.
Three other people were killed in the state, which witnessed a series of attacks by Maoists, clashes and crude bomb explosions during Thursday's vote, police told the Press Trust of India news agency.
About 32.4 million voters across Maoist-affected Jharkhand and Bihar provinces and the northern state of Haryana adjoining the Indian capital New Delhi are eligible to vote in 178 constituencies.
PTI, quoting electoral officials, said nearly 55 percent of the electorate cast their ballots in the three states. 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 claim to represent.
Bihar, where caste wars are frequent, has millions of unemployed youth and is considered to be India's poorest and most lawless state. The state averages about 2,000 murders, 1,200 robberies, 14,000 assaults and thousands of cases of rioting every year.
Orders to shoot on sight unruly supporters of candidates trying to rig votes or seize ballot boxes and overrun polling stations had been given to security personnel, the Press Trust of India news agency said.
Abductions of school children and poor security were among the issues raised by political parties in the runup to the polls, embarrassing Bihar's ruling RJD party headed by federal railways minister Lalu Yadav.
India's ruling Congress is expected to win both Jharkhand and Haryana, pre-poll surveys said.