Voting is taking place for a seat in the Lok Sabha (Lower House) of the Mainpuri Parliamentary Constituency in India's Uttar Pradesh state, and six state assembly constituencies in five states.
Two of the six assembly constituencies witnessing elections in Bharatiya Janata Party-led Uttar Pradesh are Rampur Sadar and Khatauli. Votes are also being cast in Padampur in Odisha state, Sardarshahar in Rajasthan, Kurhani in Bihar, and Bhanupratappur in Chhattisgarh state.
The Election Commission has made elaborate arrangements for the by-elections: polling is being held amid tight security measures.
In Uttar Pradesh, the Mainpuri seat is viewed as the most important one, with six candidates contesting it. The seat was left vacant after Samajwadi Party leader Mulayam Singh Yadav, the former state chief and ex-Defense Minister, died in October. His daughter-in-law Dimple Yadav is also contesting the seat.
The BJP has fielded Raghuraj Singh Shakya, once a close aide of Mulayam Singh Yadav's younger brother Shivpal Singh Yadav.
In the other two assembly seats of Uttar Pradesh, Samajwadi Party politician Azam Khan and BJP lawmaker Vikram Singh Saini were disqualified because of their conviction in separate cases.
Similarly, in Bihar's Kurhani area, ruling alliance lawmaker Anil Kumar Sahani was disqualified after being sentenced to jail in a fraud case.
In Rajasthan, the election was announced on the Sardarshahar seat after senior Congress member and lawmaker Bhanwar Lal Sharma died on 9 October after a prolonged illness.
In Odisha, the election was necessary after ruling party Biju Janata Dal lawmaker Bijay Ranjan Singh Bariha died.
Similarly, the by-election for the Bhanupratappur seat of Chhattisgarh is being held because of the death of the state's deputy speaker Manoj Singh Mandavi in November.
The result of bi-elections will be announced on 8 December.