Brisk voting was underway at 150 voting centres in the Maoist-infested Keshkal assembly seat in Chhattisgarh's Bastar district amid tight security on Monday.
Voting began at 7 am and about 20 per cent of the 146,796 voters exercised their franchise till 11 am.
Officials said nearly 5,000 policemen, including 24 companies of the Central Reserve Police Force (CRPF), have been deployed at Keshkal as the outlawed Communist Party of India-Maoist (CPI-Maoist) has a stronghold in the forested interior regions of the constituency.
A 24-seater police surveillance helicopter has been kept on standby for any emergency.
The by-election was necessitated because of the death of Bharatiya Janata Party (BJP) legislator Mahesh Baghel in a car accident in August last year. The seat is reserved for a Scheduled Tribe (ST) candidate.
While the election is expected to see a direct contest between Sevakram Netam of the BJP and Budhsan Markam of the Congress, three independents are also in the fray and one of them is backed by the Shiv Sena.
The counting will be held on February 7 at Jagdalpur, the district headquarters of Bastar. The result of the by-election result will be an indicator of where the ruling BJP and opposition Congress stand before state's 90-member assembly poll that is to be held by November this year.