Twenthy-three candidates including four each from the ruling BJP and opposition Congress are in the fray in Nasirabad (Ajmer), Weir (Bharatpur), Surajgarh (Jhunjhunu) and Kota South constituencies where 950 polling stations have been set up.
"The polling began in all the constituencies at 7 am," an election department official said.
Elaborate security arrangements have been made to ensure free and fair polling.
Paramilitary forces, companies of Rajasthan Armed Constabulary and 6,000 policemen have been deployed in all the four constituencies, according to Special DGP (Law and Order) Navdeep Singh.
There are 8,95,101 lakh electorate including 4.24 lakh women voters in the four constituencies where the bypolls were necessitated after the sitting (all from BJP) MLAs- Sanwar Lal Jat (Nasirabad), Om Birla (Kota-South), Santosh Ahlawat (Surajgarh) and Bahadur Singh Koli (Weir) - resigned from the assembly following their election to Lok Sabha.