Over 13 per cent polling was registered till 9 am in the bypoll to the Ellenabad assembly constituency in Haryana, officials said on Saturday.
The polling began at 7 am and will continue till 6 pm, they added.
The bypoll was necessitated by INLD leader Abhay Singh Chautala's resignation as MLA from the seat in January over the Centre's farm laws.
More than 1.85 lakh voters will decide the fate of 19 candidates, including INLD's Chautala, Congress nominee Pawan Beniwal and BJP-JJP candidate Gobind Kanda, in the fray for the bypoll.
A total of 211 polling booths have been set up and 121 of these have been categorised as "vulnerable" and "most vulnerable", the officials said.
Tight security arrangements have been made for the bypoll with the deployment of 30 companies of central security force, five companies of the Rapid Action Force and Haryana Police.
Gobind Kanda, brother of Haryana Lokhit Party chief and legislator Gopal Kanda, had joined the BJP recently.
Beniwal, who had unsuccessfully contested the previous assembly poll against Chautala, recently switched over from the BJP to the Congress.
Chautala, Beniwal and Kanda are locked in a triangular contest.
People, including women and elders, queued up at various polling centres to cast their votes.
Beniwal cast his vote in Darba Kalan village in the morning.
In a tweet, Chautala urged people to come out in maximum numbers and play the role of a responsible citizen by participating in this festival of democracy.
Winning the bypoll is crucial for Chautala as a loss will deal a body blow to the India National Lok Dal (INLD), which in recent years is reeling under a series of electoral setbacks.
A major part of the Ellenabad assembly constituency is rural with people mostly relying on agriculture.
Counting of votes will take place on November 2, the officials said.
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