Voting has been taking place in the Indian parliament and state assemblies for the country's 13th president.
Mrs Patil looks set to be India's first woman president
Correspondents say it is expected that Pratibha Patil will win. The only other candidate is Vice-President Bhairon Singh Shekhawat.
The poll has been overshadowed by controversy and recriminations. Mrs Patil is the candidate of the ruling Congress Party and its allies.
Opponents say Mrs Patil lacks national stature and is tainted.
She is currently the governor of Rajasthan.
Around 4,500 MPs and state legislators are eligible to vote.
The result is expected to be declared on Saturday.
The winning candidate will succeed the popular APJ Abdul Kalam.
Mrs Patil will become India's first woman president if she wins.
This is also the first time the presidential elections in India have become so controversial with some ministers expected to boycott the vote.
Many want non politicians like Mr Kalam as president
Mrs Patil has courted controversy, recently telling a Muslim congregation that the veil was introduced to protect their women from Mughal invaders, a comment she later retracted.
She also faces allegations that a bank she helped set up gave out cheap loans to her relatives before it folded up.
The Congress Party has rejected the allegations and been highly critical of the Bharatiya Janata Party (BJP)-led opposition candidate, Vice-President Bhairon Singh Shekhawat, an 83-year-old party veteran.
The Congress party has alleged that Mr Shekhawat was suspended as a policeman six days after India gained independence in 1947 for taking a bribe.
He is also accused of sheltering his son in a case of illegal farm land acquisition.
The BJP has denied the charges.
The presidency is largely a ceremonial post but with a fragmented electorate often throwing up precariously placed coalition governments, a lot will depend on a president's judgement and impartiality, the BBC's Soutik Biswas in Delhi says.