Counting of votes is due on 13 May Voting for assembly elections has begun in the southern Indian states of Tamil Nadu and Kerala and the union territory of Pondicherry.
Long queues of voters have formed outside polling centres with many coming out even before booths opened at 0700 local time (0130 GMT).
A large number of paramilitary troops and state police have been deployed to ensure the vote passes off peacefully.
The polls are part of several key state elections in India.
In the north-eastern state of Assam – where the ruling Congress party is seeking a third successive term – voting was completed on Monday.
Over the next month, polls will be held in West Bengal – where the Marxist leadership is being challenged by Trinamul Congress – a regional Congress breakaway party.
The incumbent Communists in Kerala are also facing a stiff challenge from the Congress party.
In Tamil Nadu, the ruling DMK party – a key federal ally of the Congress party – is fighting a rival regional party, the AIADMK.
More than 140 million voters are eligible to cast their votes at some 150,000 polling stations during these state polls. Counting is due on 13 May.
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