Two hours after officials began downloading 370 million ballots on Thursday, the opposition Congress and its allies are claiming victory.
With a lead in 207 seats, compared with the Bharatiya Janata Party-led (BJP) coalition's 195, it appears as though India may get a new government.
Leftist parties, which traditionally side with Congress and could decide a close race for the 545-seat parliament, also made strong gains, leading in another 48 seats.
Senior Congress leader Salman Khursheed was confident of victory. "We will form the government. There is no doubt about it."
But the BJP said it was too early to call the outcome. "These are trends, these are not results," spokesman Mukhtar Naqvi said.
Revitalised by leader Sonia Gandhi and her children, Rahul and Priyanka, the new generation of the Nehru-Gandhi dynasty, Congress surged in key states in the south and made inroads in key northern BJP strongholds.
The BJP leads the Congress in more seats, but Gandhi's allies appear to be tipping the balance. With the president appointing two members to parliament, a party must show it has the support of 272 to form a government.
The BJP's possible defeat has panicked investors, concerned about the fate of promised economic reforms that range from more privatisations to easier labour laws.