Polling has opened for the fifth phase of India's marathon elections including in two constituencies in the vote-rich state of Uttar Pradesh where opposition Congress party president Rahul Gandhi and his mother Sonia Gandhi hope to retain their seats.

More than 87 million people across seven states are eligible to vote on Monday.

Voting began amid tight security in Uttar Pradesh, where more than 25 million people are registered to cast ballots for 14 members of India's parliament.

Rahul is seeking re-election for a fourth consecutive time in Amethi. He is the son of assassinated former Prime Minister Rajiv Gandhi whose widow, Sonia, is running from neighbouring Rae Bareli.

Both constituencies are considered Congress party bastions.

India's multi-phase elections are seen as a referendum on Prime Minister Narendra Modi and his Hindu nationalist Bharatiya Janata Party, which controls Uttar Pradesh.

The seven-phase election began on April 11. The last votes will be cast on May 19 and results will be declared by May 23.

Modi campaigned on his national security record, in particular his tough stand against rival Pakistan following a suicide bomb attack by a Pakistan-based group that killed 40 Indian paramilitary police in the Indian part of the Kashmir region in February.

Uttar Pradesh is also where Hindu nationalist groups have aggressively pushed for the building of a temple on the ruins of a 16th century mosque, in the city of Ayodhya, which has become a flashpoint in tension with minority Muslims.

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