In a massive setback to Nawaz Sharif, the Pakistan Supreme Court on Friday dismissed the review petitions filed by the deposed Prime Minister and his children against the July 28 Panama Papers judgement.
There are 321,786 registered voters in the constituency who will exercise their right to vote on Sunday.
At least 44 candidates, including Begum Kulsoom Nawaz of Pakistan Muslim League-Nawaz (PML-N), Dr Yasmin Rashid of Pakistan Tehreek-e-Insaf (PTI) and Faisal Mir of Pakistan People's Party (PPP), are contesting the election.
The voting started at 8 am and it will continue till 5pm. Extra police and Rangers personnel are deployed across NA-120, with the Pakistan Army in support.
Another major step taken by the ECP for NA-120 by-poll is a pilot project of testing biometric verification machines at 39 polling stations.
State Minister for Interior Affairs Talal Chaudhry, Senator Pervez Rasheed, MNAs Shaista Pervaiz Malik, Maiza Hameed and Rukhsana Kokab, and MPA Saiful Malook Khokhar were served notices for campaigning for PML-N candidate Kulsoom Nawaz in violation of the code.
The Election Commission of Pakistan (ECP) has printed 350,000 ballot papers for the constituency's 324,786 registered voters.
Kulsoom is in London where she is undergoing treatment for her throat cancer.
Maryam, with the help of other party leaders, has run the campaign in an organised and impressive manner. The PML-N had won all 12 of the total 13 national assembly seats and 26 of 27 PA seats of Lahore in the 2013 election. She led rallies, held public meetings and conventions where she received big public response.
The NA-120 by-poll will take place on September 17.
The contesting candidates and workers of the political parties went door to door, persuading residents of the constituency to vote for them.