Neither of the two main parties - the ruling Congress party and the Hindu nationalist Bharatiya Janata Party (BJP) - are expected to win a sufficient majority by May 13 to form a government without bringing other parties in to a coalition.
The election has already been marred by militant violence, with 18 people including five poll officials and 10 security personnel killed by Maoist rebels on Thursday.
The Times of India said that militants from a Maoist Naxalite group set off a landmine under a bus carrying Border Security Force personnel in the Latehar district in Jharkhand, killing seven personnel and two civilians.
Maoists also blew up a vehicle carrying poll officials in Chhattisgarh's Rajnandgaon district, killing five, and fired shots at security officers, killing two, the paper said.
Prime Minister Manmohan Singh has called the Maoists, who claim to be fighting for the rights of tribal people and landless farmers, the greatest threat to India's stability.
The election, involving an electorate of 714 million, requires a total of six million civil and security personnel. The national Election Commission put Thursday's voter turnout at between 58% and 62%.
Manmohan Singh's Congress party faces major challenges to re-election due to the economic slowdown and a rise in militant violence. The BJP has accused the government of failure to curb terrorist attacks and ensure national security.
