The election was necessitated after the then assembly speaker G Karthikeyan died in March.
Congress candidate KA Sabarinathan was ahead of his rivals as the counting of votes in the Aruvikara assembly constituency by-election in the capital district began on Tuesday.
Counting of votes began at 8 AM and Sabarinathan took a lead of 1,322 votes in the first round.
In Saturday's polls, 77.35 per cent of the electorate had cast their votes.
The assembly constituency witnessed a triangular contest between the CPI-M, the Congress and the BJP.
The election was necessitated after the then assembly speaker G. Karthikeyan died in March. He had represented the constituency for 24 long years. Sabarinathan, 31, is his son.
There were a total of 16 candidates in the fray and 184,210 voters - up from 164,884 in 2011 when Karthikeyan won by over 10,000 votes and which saw a 70 per cent turnout.
The ruling Congress fielded Sabarinathan, while the Communist Party of India-Marxist nominated 67-year-old former speaker and minister M Vijayakumar.
The Bharatiya Janata Party put up its regular warhorse O Rajagopal, 85.
While Sabarinathan and Rajagopal kept a tab on the counting process from their homes, Vijayakumar was spotted at the counting centre in the capital city.