Candidates of the ruling Congress-led United Democratic Front, Communist Party of India-Marxist-led Left Democratic Front and the Bharatiya Janata Party are engaged in three-cornered fight in the rural constituency, located about 15 km south-east of the capital.
According to official sources, impressive voter turnout was reported from most parts of the rural segment in the initial hours itself.
A total of 1.6 lakh voters are eligible to cast their ballots in the polls.
Though the main battle in Neyyatinkara is between R Selvaraj of the Congress and CPI-M's F Lawrence of the LDF, entry of former Union Minister and senior BJP leader O Rajagopal has virtually turned the contest into a three-cornered one.
The by-poll is especially crucial for CPI-M as it is their sitting seat, vacated by R Selvaraj, who has joined the Congress and is now contesting on the 'hand' symbol.
The poll also came at a time when the CPI-M is on the defensive with several of its activists being arrested in connection with the murder of Revolutionary Marxist Party leader T P Chandrasekharan at Onchiyam in Kozhikode on May 4.