Hindu nationalist opposition candidate Narendra Modi held a strong early lead in the race for prime minister.
And his Bharatiya Janata Party (BJP) is forecast to win the biggest chunk of the 543 parliamentary seats up for grabs, although they are expected to fall just short of a majority.
That will likely result in some form of coalition government, according to observers.
Elderly women in saris and young men in polo shirts and jeans lined up to vote in Dibrugarh, a river town in the state of Assam.
It was one of two states to vote on Monday.
Some 815 million Indians are registered for the election. Results from all 935,000 polling stations are expected on May 16.