Polling began on a peaceful note in the Tiruvananthapuram Lok Sabha by-election at 0700 IST on Friday.
Long queues were seen at polling booths even before voting began.
There are 1,055 polling stations for the nearly 1.2 million electorate. Electronic voting machines are being used in the polls.
The main fight is between Congress candidate V S Sivakumar, fielded by the ruling United Democratic Front, and the Communist Party of India nominee Pannian Ravindran of the Opposition Left Democratic Front.
Bharatiya Janata Party's former state president C K Padmanabhan is also in the fray.
Counting of votes will be on November 22.
The election was necessitated following the death of CPI leader and former Chief Minister P K Vasudevan Nair.