Two Assam assembly constituencies go to polls on Saturday amid tight security, officials said.
An Election Commission official said voting for the Dhekiajuli and South Salmara seats would begin at 7 a.m.
The by-elections were necessitated after incumbent legislators Joseph Toppo from the Asom Gana Parishad (AGP) and Badruddin Ajmal of the Asom United Democratic Front (AUDF) were elected to the Lok Sabha in May.
Around 161,959 voters will decide the electoral fate of seven candidates in Dhekiajuli with the fight for political supremacy expected between the AGP and the Congress.
Dhekiajuli is in north Assam's Sonitpur district, about 160 km from here.
Three independent candidates contesting the polls in Dhekiajuli late on Thursday announced their support to the Congress.
"We are confident of winning the seat," Congress candidate Bhimananda Tanti told IANS.
In the South Salmara seat, the contest is expected to be a direct one between the AUDF and the Congress party.
The AUDF has fielded Abdur Rahman Ajmal, son of party chief Badruddin Ajmal, while former minister Wajed Ali Choudhury is the Congress candidate.
This is the first time Ajmal's son, a 28-year-old Islamic cleric from Deoband, has entered the electoral fray.
A total of 131, 836 voters are eligible to exercise their franchise in South Salmara with five candidates in the running.
South Salmara is in west Assam's Dhubri district, about 300 km from here.