Voters were seen standing in queues even before the polling began at 8 a.m. in most of the 5,413 polling stations in these constituencies spread over 12 districts. The polling will continue till 5 p.m.
The Election Commission has made elaborate arrangements to ensure free, fair and peaceful polls, billed as the semi-final before the 2014 elections.
About 50,000 security personnel, mostly central-armed reserve police, were deployed while almost an equal number of polling staff were on duty.
Over 46 lakh voters will decide the political fortunes of 255 candidates, 13 of them in the Nellore Lok Sabha constituency.
The counting of votes will be taken up Friday.
Most of the constituencies are witnessing triangular contests among the ruling Congress party, main opposition Telugu Desam Party (TDP) and YSR Congress Party.