The elections in Bihar state and in neighbouring Jharkhand on Tuesday are expected to boost the strength of the country's ruling Congress party-led coalition.
But the blasts in Bihar, one of India's most populous but least developed and most lawless states, did not deter voters and an estimated 30% of them had cast their votes by 2pm (0380 GMT), officials said.
"We have received reports of blasts in seven polling booths in Munger district," said Bihar's police chief Narayan Mishra, referring to premises where voting takes place.
"Police in the area has been instructed to increase security around the booths," he said.
State officials said the blasts were triggered by gangs trying to create panic and influence the vote.
As well as Bihar, voters were also electing a new state assembly in Jharkhand state, where police have deployed helicopters and mine-resistant vehicles to provide security.
"Police in the area has been instructed to increase security around the booths"
Bihar police chief
Seventeen people were killed in Bihar and Jharkhand during the first phase of voting on 3 February.
Leftist rebels, criminal gangs and private forces in Bihar regularly resort to violence, particularly during elections in a bid to sway the outcome.
India's third most populous state with 38 million people, Bihar is also notorious for its caste and religious divisions, and has long been blighted by religious clashes.