Lawmakers across India voted on Monday to elect the country's 14th President with Union Minister M Venkaiah Naidu declaring that ruling NDA candidate Ram Nath Kovind would win with a "comfortable and respectable margin" against opposition-backed Meira Kumar.

Polling to choose the successor of incumbent Pranab Mukherjee, who demits office on July 25, started simultaneously in Parliament in the national capital and state assemblies at 10 am.

All elected MPs and MLAs are eligible to cast their vote through secret ballot.

As voting picked up, many MPs, including women members, were seen queuing up in Parliament House outside the polling booth in Room No 62 for the voting, that closes at 5 pm.

Prime Minister Narendra Modi and BJP chief Amit Shah were among the first to cast their votes in Parliament.

Modi, clad in a beige colour sleeveless jacket over a white kurta-pyjama, reached the Parliament premises early to vote on Monday, also the first day of the over three-week monsoon session of Parliament.

The Prime Minister said the session was expected to herald in a new hope for India. "Today the monsoon session begins. Like the monsoon brings hope, this session also brings the same spirit of hope."

Shah, who is an MLA from Gujarat, was allowed to cast his vote in Parliament.

Congress President Sonia Gandhi and her son Rahul also voted.

In state assemblies, brisk voting was on as MLAs and some MPs also queued up outside their legislative complexes to choose the next President.

Polling began on a brisk note in Kerala, Tamil Nadu, Maharashtra, Telangana, Andhra Pradesh, Odisha, Bihar, Uttar Pradesh, as tens of hundreds of legislators voted in the first three hours of polling.

Delhi Chief Minister Arvind Kejriwal led his the AAP lawmakers to vote.

In Punjab, Chief Minister Amarinder Singh had called all Congress legislators in the state for breakfast before the voting. The Congress legislators later gathered at the Punjab Bhawan here for a meeting on the issue.

In the Haryana Assembly, ruling Bharatiya Janata Party (BJP) legislators Kamal Gupta and Latika Sharma were among the first voters.

Tamil Nadu Chief Minister K. Palaniswami was among the first voters at the assembly complex in Chennai. He was followed by Speaker P. Dhanapal, DMK's M.K. Stalin and former Chief Minister O. Panneerselvam.

The Election Commission has put up a total of 32 polling stations — one in Parliament House Room No 62 and rest one each in state legislative assemblies — for the polls.

A total of 776 MPs and 4,120 MLAs are eligible to vote. The total value of votes of the electoral college is 10,98,903, and the NDA candidate is slated to cross the halfway-mark and get about 63 per cent votes.

The ballot boxes will be brought to Delhi for counting on July 20. Results will be declared that day.

Kovind — ex Bihar governor — has an advantage over Meira Kumar — a former Lok Sabha speaker — as the numbers are stacked in favour of the ruling NDA.

The BJP and allies have nearly 63 per cent of the vote while the Congress-led opposition has a little over 35 per cent. Independents and other smaller parties, who have not made their choice known, have two per cent.

If elected, Kovind will be the second Dalit President of India. K.R. Narayanan was the first from the community to hold the post.

"Kovindji will win with a comfortable and respectable margin," Information and Broadcasting Minister Venkaiah Naidu said before voting. 

Meira Kumar said she has requested members of the electoral college to "heed the inner voice of their conscience" and vote in the best interest of the country.

"This ideology is of social justice, inclusiveness, secularism, transparency, freedom of expression, freedom of press and total destruction of the caste system. This is the ideology which binds India together. So it is very important that we protect and preserve it," said Kumar.

Her son Ansul said it was not a personal fight "but a fight of ideologies".

"The forces which are present should be decimated. If it's just about numbers then why are we going through the whole process. The winner can be announced earlier based on the parties' support. We are excited… We are part of it. I have travelled with my mother," Ansul Kumar said.