A close contest is expected between Begum Kulsoom Nawaz, wife of former prime minister Nawaz Sharif, and Pakistan Tehreek-e-Insaf (PTI) candidate Dr Yasmin Rashid.
The NA-120 seat fell vacant after the disqualification of Nawaz Sharif as the prime minister by the Supreme Court on July 28 in the Panama Papers case.
Analysts say Sunday's by-poll will be a key indicator as to whether Sharif's dismissal has led to an erosion of support.They expect Sharif to hold onto the constituency, but feels the poll is an important bellwether for upcoming national elections in mid-2018.
The polling will start at 8 am and conclude at 5 pm without any break.
Earlier, the ballot papers were dispatched to all 220 polling stations in the constituency under supervision of Pakistan Army.
Although over two dozen candidates are in the running, it is believed that the main contest is between Pakistan Tehreek-e-Insaf's Dr Yasmin Rashid and PML-N's Kulsoom Nawaz.
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The report revealed that the support staff including bus drivers and conductors were using toilets meant for students. Two wounds were found on his neck, and it was the second one that caused his death, the report added.
Tight security through Army, Rangers and the police has been ensured in the constituency while CCTV cameras are also fixed where necessary.
The PTI and the PPP had questioned validity of over 29,607 votes in the electoral rolls on the basis that the thump impressions of the voters were not present in Nadra record - making it impossible to verify those at the time of balloting.
Reacting to the ruling, 43-year-old Maryam, a leader of the Pakistan Muslim League-Nawaz, said revenge was taken in the name of accountability.
Kulsoom is in London where she is undergoing treatment for her throat cancer.
According to final polling scheme for NA-120 by polls, the ECP has set up 220 polling stations, out of which 103 are for men, 98 for women and the remaining 19 are combined. "Do you accept this unjust verdict?" she asked her supporters, adding, "If you are against the verdict, then vote for lion on 17 September".