Kalsoom Nawaz managed to win the election from northeastern Lahore city - a stronghold of her husband's Pakistan Muslim League (PML) - after garnering over 59,000 votes.
In light of the former first lady's ongoing medical treatment in London, her daughter Maryam is spearheading the campaign in the constituency.
The Coalition for Election and Democracy (CED), an alliance of registered independent civil society organisations, in its exit-poll survey showed a slender lead for Nawaz.
Voters were expected to turn up in good numbers at the polling stations in view of the intense mobilisation campaign by the prominent candidates.
In the 2013 election in the constituency, the PML-N received a majority of almost 40,000 votes ahead of their closest rivals from the PTI party, led by Imran Khan.
The election to the parliamentary seat is seen as a test for the PML-N party also because it comes ahead of the general election in Pakistan next year.
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The voters' turnout that was relatively low in the morning started picking up in the afternoon. The Election Commission of Pakistan has set up 220 polling stations for the voters.
According to the unverified and unofficial results being shared by DawnNews, a close contest is being observed between PTI's candidate Dr Yasmin Rashid and PML-N's Kulsoom Nawaz - with no one claiming a considerable lead as yet.
Imran Khan asked the residents of NA-120 to cast their votes and give verdict against the "Godfather" (Nawaz Sharif).
A five-judge SC bench headed by Justice Asif Saeed Khosa had taken up the petitions seeking review of the court's ruling which disqualified Sharif and ordered the filing of corruption cases against him and his children - Hussain, Hassan and Maryam Nawaz, son-in-law retired Captain Mohammad Safdar and Finance Minister Ishaq Dar.
The by-election for a National Assembly seat in Lahore, that fell vacant with Sharif becoming ineligible to remain a legislator, has brought in the open the split in the party between his daughter, Maryum, and his younger brother, Shahbaz Sharif, who is now the CM of Punjab.