Brisk polling was witnessed in the early hours with M K Alagiri of DMK, opposition BJD MPs led by B Mahtab and Pratap Singh Bajwa of the Congress among the early voters.
Prime Minister Manmohan Singh is expected to cast his ballot at 11 AM.
As many as 4,120 MLAs and 776 MPs with a total vote value of 10.98 lakh votes are eligible to cast their ballot in the 14th Presidential election.
Members of the Rajya Sabha, the Lok Sabha and state legislative assemblies make up the electoral college for the Presidential election.
The run-up to the Presidential poll saw its twists and turns with key UPA ally Trinamool Congress first declaring its opposition to Mukherjee only to take "the painful decision" to back him.
Besides the UPA and its four supporting parties -- SP, BSP, RJD, JD(S) -- NDA members JD(U) and Shiv Sena as also the CPI(M) and Forward Bloc have declared their support for Mukherjee.
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The CPI, RSP, TDP and TRS, which together have about 36,000 votes, are likely to abstain from voting.
There are around 32,000 votes with smaller parties who have no presence in Parliament but have legislators in the states.
Sangma, a former Lok Sabha Speaker, jumped into the fray playing the tribal card with initial support of the BJD and AIADMK. The BJP and Akali Dal decided to back Sangma, who quit the NCP, the party he founded with Sharad Pawar, to contest the election.
With the support of SP (66,688 vote value), BSP (45,473), JD-U (40,737), Shiv Sena (18,495), JD-S (6,138) and CPI-M and Forward Bloc (around 41,000), Mukherjee can expect vote share of around 7.5 lakh.
In the electoral arithmetic, Sangma is expected to be way behind Mukherjee and is likely to garner around 3.15 lakh vote value. (More)