The main challenge was how to authenticate voters on-chain. I overcame it by authenticating them off-chain as below:

Voters will go on the website and enter their voterId no. and then OTP will be sent to their attached mobile numbers as per the records maintained by the government of India.
After authentication, the user will be minted with an NFT proving the voter is registered. Authentication according to area wise can be filtered using records maintained.

For voting voters will be again authenticated and they will be signing the tokenId which they were minted and the candidateId to which they want to vote so that it can be authenticated on-chain.

tokenIds or NFTs are keccack256 hash of voterIds so that it cannot be determined by address that which ID it belongs to, which hides voter's identity on-chain.