Voting for the by-elections to 15 Assembly constituencies and one Lok Sabha seat in six states of the eastern region began on Monday amid tight security.
Of the 15 Assembly seats, five are in Bihar followed by four in Assam, three in Sikkim and one each in Arunachal Pradesh, Odisha and Meghalaya.
The lone Lok Sabha seat going to the by-polls is in Bihar.
Sikkim Chief Minister Prem Singh Tamang is among the prominent candidates in the fray.
Voting is taking place in Samastipur (SC) Lok Sabha seat besides Kishanganj, Nathnagar, Belhar, Simri Bakhtiyarpur and Daraunda Assembly constituencies in Bihar.
More than 30 lakh voters are eligible to exercise their franchise to elect from among 51 candidates in Bihar.
By-elections to four Assam constituencies - Rangapara, Sonari, Ratabari and Jania - will decide the fate of 20 contestants.
Some 6,78,898 voters, including 3,27,803 female and one of the third gender, are eligible to exercise their franchise.
In politically-sensitive Bijepur Assembly seat in Odisha, the election is being held in 285 polling stations, of which 129 are identified as sensitive, where 2,32,264 people can exercise their franchise.
By-poll to the seat was necessitated after Chief Minister Naveen Patnaik, who had won from two segments in the Assembly polls held early this year, resigned from Bijepur in Bargarh district and retained his home turf Hinjili in Ganjam district.
Besides Poklok Kamrang, where the Sikkim chief minister is in the fray, by-polls are being held in Martam Rumtek and Gangtok Assembly seats. Six contestants are in the fray in each of the constituencies.
Former Indian football team captain Bhaichung Bhutia is contesting from Gangtok on Hamro Sikkim Party (HSP) ticket.
The Khonsa West Assembly seat in Arunachal Pradesh is witnessing a straight contest between two independent candidates - Chakat Aboh and Azet Hamtok.
Chakat Aboh, the wife of NPP leader Tirong Aboh who was shot dead in May, has been unanimously fielded by five major political parties in the state.
By-polls to the Shella Assembly constituency in Meghalaya's East Khasi Hills district is being held in 58 polling stations where 29,763 votes, including 14,809 males and 14,954 females -- can exercise their franchise.
The by-poll was necessitated due to the death of Donkupar Roy, who was the MLA of Shella since 1988 and also the Speaker of the 10th Meghalaya Assembly, on July 28.
Six candidates, including Donkupar Roy's son Balajied Synrem, are in the fray.