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.
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 LK Advani and Sushma Swaraj also cast their ballot. UPA managers are confident that 75-year-old Ansari will get at least 500 votes in a contest giving him a distinct edge as far as the arithmetic is concerned.
The voting began at 10 AM and will continue till 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 RPN Singh and V Narayanasamy.