Voting is underway in 41 constituencies in the Hindi heartland states of Uttar Pradesh and Bihar, and in the eastern state of West Bengal, in the last stage of polling in a nine-phase election that started five weeks ago.
Voters started queueing up outside polling booths across the three states as Bharatiya Janata Party's (BJP) prime ministerial candidate Narendra Modi tweeted early in the morning to urge people to "vote in large numbers".
"My special request to the youth - go out & vote and take your family and friends along to the polling booth,'' tweeted Mr Modi, 63, who is contesting the election from Varanasi in the populous state of Uttar Pradesh.
Varanasi is the most high profile of the 41 constituencies going to the polls on Monday. It pits Mr Modi, three-time Gujarat Chief Minister, against Mr Arvind Kejriwal, the 45-year-old chief of Aam Aadmi Party. The third candidate in Varanasi is Mr Ajai Rai, 41, a local politician from the ruling Congress Party.
While Mr Modi is expected to cruise to a win, Mr Kejriwal is likely to cut into his margin of victory.
Though temperatures have soared in the summer months of April and May, voters have been turning out in record numbers since April 7 in the election characterised by a bitter campaign waged by the BJP and the Congress Party, the two main national parties.
Around 814 million were eligible to vote for the 543 seats in the Lower House of Parliament, with over 100 million first-time voters who have been wooed by all the main parties through social media platforms like Facebook and Twitter.
Exit poll results will be announced on Monday night after voting concludes and will give a glimpse of how the elections will stack up for the main parties. While results are expected only on Friday (May 16), the jostling for allies is expected to start soon.
The ruling Congress Party is expected to perform poorly after a decade in power, while the BJP is expected to win the largest number of seats but fall short of the 272 seats needed to secure an outright majority.
The BJP's success will also depend on its performance in the Hindi heartland states of Uttar Pradesh and Bihar, which together account for 120 seats.
Polling in the two populous and poor states has taken place over six phases.
In Uttar Pradesh, regional parties like the Bahujan Samaj Party and the Samajwadi Party dominated past elections. But this time, the BJP has run a high-stake campaign with Mr Modi - who has never fought elections outside of Gujarat state - deciding to stand from Varanasi, a spiritual centre for Hindus.
Uttar Pradesh, in particular, has helped elect several prime ministers, including BJP's Atal Bejari Vajpayee, and members of the Gandhi-Nehru dynasty.
The BJP's campaign has been particularly aggressive in western parts of Uttar Pradesh where in September last year, 43 people were killed in Hindu-Muslim riots in Muzaffarnagar, with Mr Modi's key aide Amit Shah forced to apologisie for making a communally charged statement.
Muslims in India are concerned about the rise of Mr Modi who is seen to have failed to control riots in Gujarat in 2002 in which more Muslims than Hindus were killed. The taint of the riots has not left Mr Modi even though in the election, he has focused on the agenda of development and strong good governance.