In all, over 80,000 security personnel have been deployed.
The political fate of 239 candidates is being decided Thursday.
Among these are Sachin Pilot, C.P. Joshi, Girja Vyas, Chandresh Kumari and Jitendra Singh of the Congress, and Sanwar Lal Jat, a minister in Vasundhara Raje government, and Olympic silver-medalist shooter Rajyavardhan Singh Rathore of the BJP, and Jaswant Singh and Buta Singh who are contesting as Independents.
The Congress and the BJP are fighting for all 20 seats, while the Aam Aami Party is contesting 17, and the Bahujan Samaj Party 18 seats.
The state is witnessing a keen tussle between the BJP and the Congress. In the last Lok Sabha polls, the Congress won 20 seats and the Bharatiya Janata Party four, and one seat was won by an Independent.