A total of 788 Lok Sabha and Rajya Sabha MPs, including nominated members, are eligible to vote in the poll to elect the 14th Vice President.
Ansari, who is seeking re-election, has the support of the UPA, its allies, the Left parties as also other parties supporting it from outside.
Jaswant Singh, a former Minister of External Affairs and Finance, has the support of the NDA and AIADMK.
Prime Minister Manmohan Singh, UPA chairperson Sonia Gandhi and Rahul Gandhi came together to cast the ballot in the Vice Presidential election.
BJP leaders L K Advani and Sushma Swaraj also cast their ballot.
The voting began at 10 am and will end at 5 pm. Counting of votes will be taken up at 6 pm.
Among the early voters were Congress MPs Priya Dutt and Abhishek Manu Singhvi and Union Minister R P N Singh and V Narayanasami.
It may be recalled that Jaswant Singh was in Chennai yesterday and called on Tamilnadu Chief Minister and AIADMK general secretary J Jayalalithaa to seek her support.
After the meeting, Jayalalithaa said, 'If we are to have a true democracy, there has to be an opposition in the country. Simply because the candidate put up by the ruling dispensation appears to have a winning chance, it does not mean that no one else should contest this election'.
The AIADMK chief said her party's backing to Singh was mainly to 'reinforce my strong belief, principle and conviction (that) in a democracy, there has to be an opposition.'
