Polling began on Monday for the by-election to Madhya Pradesh's Jhabua Assembly seat.
The voting, which started at 7 am across 356 booths, will end at 5 pm, an official said.
The ruling Congress in the state is making all efforts to wrest the tribal-dominated seat from the opposition BJP.
BJP leader G S Damor defeated Vikrant Bhuria of the Congress in the Assembly polls last year.
Later, Damor contested the Lok Sabha election from Ratlam seat where he defeated Vikrant Bhuria's father Kantilal Bhuria. He subsequently resigned as MLA from Jhabua, which necessitated the by-election.
The main contestants for the bypoll are senior Congress leader Kantilal Bhuria and BJP's Bhanu Bhuria, who is making his electoral debut.
Three independents - Kalyan Singh Damor, Neelesh Damor and Rameshwar Singhar - are also in the fray.
The constituency has 2.77 lakh voters, including three from the third gender, the official said.
Four companies each of the Central Industrial Security Force (CISF) and State Armed Forces (SAF), besides 600 police personnel from other districts have been deployed at the polling booths, he said.
The CISF personnel have been deployed at 61 booths declared critical by the Election Commission, he added.
The Congress, which enjoys a slim majority in the Assembly, is trying its best to wrest the seat from the BJP.
Earlier, the Congress won the seat, reserved for tribals, 10 times since 1952. But, the BJP wrested it in 2013 and retained it in 2018.
During the campaigning, the BJP attacked Kantilal Bhuria and the Congress, saying they did nothing in terms of the development of the region in the last three decades.
It also highlighted Bhanu Bhuria's 'clean image'.
On the Congress' side, Chief Minister Kamal Nath led the charge and several state ministers also held rallies. Party veteran Digvijaya Singh also canvassed for Kantilal Bhuria, known to be his loyalist.
However, Congress general secretary Jyotiraditya Scindia stayed away from campaigning.
From the BJP, former chief minister Shivraj Singh Chouhan and other leaders, including state party chief Rakesh Singh and Union ministers Narendra Singh Tomar and Faggan Singh Kulaste, campaigned for the bypoll.