Simultaneous by-poll is being held in the Kotulpur assembly constituency of Bankura district.
"Over 24 percent votes were cast till 9 am," an official said here.
An electorate of over 88.77 lakh, including 42 lakh women, is eligible to vote at 11,321 polling stations. Elections are taking place in Jhargram, Medinipur, Purulia, Bankura, Bishnupur and Asansol.
The political fate of 72 candidates will be decided. Eleven of the contestants are women.
The Trinamool, Left Front, Congress and the Bharatiya Janata Party are in fray in all the parliamentary constituencies.
Of the Left Front partners, the Communist Party of India-Marxist (CPI-M) is contesting four seats, leaving one each to the Communist Party of India and All India Forward Bloc.
In 2009, Left Front candidates had bagged all the seats.
The Asansol seat has generated interest among poll watchers where BJP candidate and Bollywood singer Babul Supriyo is giving a tough fight to Trinamool's trade union president Dola Sen and sitting MP CPI-M's Bansa Gopal Choudhury.
A stiff contest is in Bankura where nine-time CPI-M MP Basudeb Acharia is being challenged by Trinamool star candidate and legendary Bengali film heroine Suchitra Sen's actress daughter Moon Moon Sen.
The focus is also on Jhargram - for long the epicentre of Maoist violence, and Purulia and Bankura, parts of which are affected by the Left extremism.
Security had been beefed up in the Maoist heartland ahead of the polls
As many as three helicopters, with a medical support team onboard, have been pressed into service in the affected areas. Besides choppers, as many as 10 satellite phones are being used.
Central security forces have been deployed across all booths in the Maoist belt.
Bomb disposal squads, road opening party and anti-landmine vehicles too have been deployed in the Maoist areas.
Meanwhile, the CPI-M has alleged that polling agents were beaten up or abducted in several areas. The Trinamool has denied such claims.