According the initial report, the Telangana Rashtra Samiti (TRS) is leading in 11 constituencies and the Bharatiya Janata Party (BJP) has acquired one.
Early trends show the two parties may retain their seats while the TRS has also established lead in one constituency held by the Telugu Desam Party (TDP).
The results from seven constituencies, where electronic voting machines (EVMs) were used, are likely to be announced before noon of Jul 30.
However, it may take some more time in the remaining five constituencies where ballot papers were used.
The by-polls were necessitated by the resignation of all 10 legislators of the TRS and one each of the BJP and the TDP on the issue of a separate Telangana state in 2010.