The polling, which began at 7 a.m. at all the 302 polling stations in the constituency in Suryapet district , will continue till 5 p.m.
A total of 2,36,842 voters are eligible to exercise their franchise in the by-election, the first since Telangana Rashtra Samithi (TRS) retained power in the state in December 2018.
While 28 candidates, including three women are in the fray, the main contest is between TRS candidate S. Saidi Reddy and opposition Congress party's N. Padmavati Reddy, wife of state Congress chief Uttam Kumar Reddy.
The by-poll was necessitated by the resignation of Uttam Kumar Reddy from the Assembly following his election to Lok Sabha from Nalgonda.
The seat has never been won by the TRS since it was created in 2009 during the delimitation of constituencies. Uttam Kumar Reddy of Congress won all three elections (2009, 2014 and 2018).
Bharatiya Janata Party (BJP), which has fielded Kota Rama Rao, is looking to increase its vote share while Telugu Desam Party (TDP), which is battling to maintain a toehold in Telangana politics, has fielded Chava Kiranmayi.
Saidi Reddy, who had lost to Uttam Kumar Reddy 7,466 votes in 2018, cast his vote at Gundlapalli village.
Police confined Uttam Kumar Reddy's to his house in Huzurnagar, following a complaint to the election authorities by the TRS. The ruling party said since the state Congress chief is a non-local, he should leave the constituency. Uttam Kumar Reddy, however, argued that he is a local as he has his house in the town.
Tension prevailed at Uttam's house Sunday night when officials reached there. However, district collector and returning officer Amoy Kumar later clarified that the Congress leader can remain in the constituency as Huzurnagar is one of the Assembly segments in his Nalgonda constituency, but he will not be allowed to come out of his house.
Authorities have made elaborate arrangements for the peaceful and smooth conduct of the by-poll. The Election Commission has deployed nearly 1,500 personnel and deployed 965 Electronic Voting Machines (EVMs) .
Officials said about 1,500 police personnel were deployed in the constituency as part of the security arrangements to ensure free and fair poll.
( With inputs from IANS )