The polling which began at 8 am local time will continue till 5 pm without any break.
In all, 522 candidates are contenting for 15 National Assembly and 26 Provincial Assembly constituencies.
The Election Commission has set up 7,606 polling stations across the country with around 1,800 sensitive and over 1,500 highly sensitive polling stations, where personnel of armed forces will be deployed.
There are around 8.5 million registered voters in these constituencies.
Most of the seats had been vacated by the winners of more than one seat, while others had fallen vacant because of disqualification of candidates by superior courts.
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The Election Commission has postponed election for Dera Ismail Khan-cum-Tank constituency due to security concerns, Radio Pakistan reported.
The banned Tehreek-i-Taliban Pakistan (TTP) has warned women of Hangu in Khyber Pakhtunkhwa province not to take part in today's by-election, otherwise they will be kidnapped and killed, the Dawn reported.
The polling will be held for one seat of Khyber Pakhtunkhwa Assembly.
The threat by the Taliban was conveyed through leaflets found by people in Hangu bazaar yesterday.
In most of the constituencies, a neck and neck contest in expected between the ruling PML-N and
Political observers here say that the ruling party is likely to have an upper hand.
The Pakistan Peoples Party, which was defeated in the May 11 General Elections, is not seen by many as a challenge to either the PML-N or Imran's party.
The PML-N and the PPP have not fielded any candidate for any provincial seat in Khyber Pakhtunkhwa.
Similarly, no candidate of the PML-N is contesting for any provincial assembly seat in Sindh.
There are 201 candidates contesting for National Assembly constituencies across the country, 180 candidates for 15 Punjab Assembly constituencies, 82 candidates for four Sindh Assembly's constituencies and 27 and 32 candidates for four each constituencies of Khyber Pakhtunkhwa and Balochistan Assembly constituencies respectively.