That is roughly 900 million of India's 1.3 billion people, according to the Election Commission, the body tasked with overseeing the monumental endeavour.
Some observers have billed this as the most important election in decades and the tone of the campaign has been acrimonious.
Election Commission of India (ECI) said it has made adequate arrangements and security measures to ensure a free and fair polling in the polling areas.
And Modi's critics say his party's Hindu nationalism has aggravated religious tensions and violence against Muslims and other minorities in constitutionally secular India.
Senior Congress leader Ahmed Patel also took a jibe at Prime Minister Modi, saying "the true colours are out" and termed the statement from Mr Khan as "mahamilavat", a phrase used by Modi to describe the opposition's "mahagathbandhan" - a grand alliance of all opposition parties to defeat him. His grandmother, Indira Gandhi, was assassinated by her bodyguards while prime minister. His sister, Priyanka Gandhi, formally joined politics in January.
Hundreds of voters began to queue up outside polling centres early Thursday morning. "Modi-ji's invincibility will be in full view in this election", Gandhi said in Uttar Pradesh's Raebareli constituency, from where his mother Sonia Gandhi filed her nomination papers on Thursday. They trek through the jungle to bring voting machines to a man who lives alone in a lion-infested forest in the western state of Gujarat, and climb to almost 15,000 feet above sea level to set up a polling station for 12 residents of a village in the northern province of Leh.
The election panel said voters were turning out in large numbers in the forested eastern state of Chhattisgarh, where Maoist insurgents were blamed for a bomb attack that killed a BJP state legislator and four security officials on Tuesday.
Phase one on Thursday sees some 142 million people able to cast ballots. Seats in 20 states will be decided on the first of seven days of voting, with the entire elections spread over six weeks.
In parts of the country bordering Pakistan sentiment has ever been strong because of the bloodshed during the nations' partition in 1947 and three wars since then.
Supporters of the Congress party are 21 percentage points more likely than backers of the BJP to voice the view that job opportunities have worsened-a partisan sign of how those who care about these issues may vote.
Alongside with the general election, the state Legislative Assembly elections will be held in Andhra Pradesh, Arunachal Pradesh, Odisha and Sikkim.
"I support the prime minister's policies, especially his foreign policy", Sachin Tyagi, 38, a mobile phone shop owner, told Reuters near a polling station in the northern state of Uttar Pradesh. Voters also care about national security, after India and its archrival Pakistan - both nuclear powers - exchanged airstrikes this winter.
Hundreds of millions of Indians have escaped poverty since the turn of the millennium but huge challenges remain.
But India's growth hasn't meant a better employment outlook in the country, where an estimated 1 million people join the labor pool each month.
Mr Modi's government has been accused of hiding uncomfortable jobs data. It also promises an income subsidy program for the poorest families and for farmers.
Aerial clashes between the nuclear-armed neighbours followed a suicide attack in February by a militant group based in Pakistan that killed 40 Indian paramilitary police in Kashmir. The argument that Khan is trying to subtly help the opposition would go with the official narrative of the ruling Pakistan Tehreek-e-Insaf (PTI), which has called out the BJP for its "anti-Muslim" alignment.