Elections are under way in Lahore's NA-122 and PP-147 constituencies, and Okara's NA-144 seat.
Army and Rangers personnel have been deployed at both Lahore and Okara polling stations declared 'sensitive'.
All arrangements for today's elections had earlier been finalised.
There are nearly 350,000 registered voters in Lahore and 316,000 in Okara.
Of the 284 polling stations in Lahore, there are 134 each for men and women, while 16 polling stations are for both genders. The entire constituency has been declared 'sensitive', but 12 polling stations have been labelled 'highly sensitive' and more security personnel have been deployed here.
In Okara, there are 210 polling stations, 25 of which have been declared 'sensitive'.
Polling is to begin at 8:00am and is expected to continue uninterrupted till 5:00pm.
PTI polling agent temporarily suspends polling at NA-122's Garhi Shahu polling stations
Polling at stations 96 and 98 was temporarily suspended, amidst fears of rigging, by a PTI polling agent.
While speaking to Media, he said "The ballot boxes were not sealed in front of us, the ballot papers were not checked in front of us, and when Ayaz Sadiq came, they quickly started the balloting while only one polling agent of ours was inside."
"As per election commission rules, all boxes have to be attested and sealed in front of a polling agent. They did everything themselves. When Ayaz Sadiq came, they immediately started polling," he claimed.
"We think polling should be suspended until the matter is resolved, otherwise there might be rigging," he said.
An application was submitted to the presiding officer detailing the matter and polling resumed shortly after.
'Will accept result, whatever it is', says Ayaz Sadiq
PML-N candidate Ayaz Sadiq speaking to the media early on Sunday said his party would "accept the result, whatever it is".
"Hopefully it will be in our favour," he added.
The former National Assembly speaker said his morale was high. "I am very jubilant, very happy, very focused, and very confident," he told reporters.
"Look at my camp and look at other party's camps ─ you will see the difference," he said. "Our polling agents reached our camps at 7:00am, despite polling beginning at 8:00am."
"We have high hopes for the contest," he said.
The PML-N has also asked activists not to engage in fights and arguments, Ayaz said. "Calmly cast your votes. If people are fighting, instead of taking the law into your hands, inform the police," he said.
Ayaz Sadiq cast his vote at the Garhi Shahu polling station.
The much-awaited by-election in Lahore’s NA-122 constituency ─ the ‘Takht-i-Lahore’ ─ is where former National Assembly speaker, Sardar Ayaz Sadiq of the PML-N, faces off against Aleem Khan of the Pakistan Tehreek-e-Insaf (PTI).
Re-election for Okara's NA-144 constituency is mainly a fight between the ruling PML-N candidate, Ali Arif Chaudhry, and independent candidate Riazul Haq Juj. The election will also be contested by PTI's Ashraf Sohna and PPP district president and former MNA Chaudhry Sajjadul Hasan
Both NA-122 and NA-144 fell vacant when an election tribunal de-seated Ayaz Sadiq and Arif Chaudhry.
The contest for the provincial assembly seat, PP-147, is mainly between PML-N’s Mian Mohsin Latif and PTI’s Shoaib Siddiqui.
NA-122: 'Takht-i-Lahore'
Ayaz Sadiq had been elected to the NA from the same constituency in the 2013 general election, having defeated PTI Chairman Imran Khan. But on Aug 22 this year, an election tribunal deseated Ayaz Sadiq, declaring the 2013 election in the constituency null and void, and ordered a re-election.
The decision had come following a petition by runner-up candidate Imran Khan, who had challenged the 2013 election result on grounds of rigging.
While the PML-N leader has challenged the tribunal’s decision in the Supreme Court on grounds that the “election tribunal’s impugned order was against facts and relevant laws,” his party had decided to contest the by-poll in light of the election tribunal’s order.
The by-poll has gathered a lot of hype not just because of the high-profile personalities involved but also because NA-122 was among the four controversial constituencies — the other three being NA-110 (Sialkot), NA-125 (Lahore) and NA-154 (Lodhran) — where PTI had demanded for voters’ thumbprint verification in the hopes of determining the legitimacy of the 2013 general election.
It is pertinent to mention here that other than NA-110, election tribunals in the remaining three constituencies (NA-125, NA-122, NA-154) had also declared the respective elections as null and void, ordering re-polls.
Polling will start at 8:00am and will continue without interruption till 5:00pm under strict supervision of Pakistan Army along with hundreds of policemen and other law enforcement agencies.
Ayaz Sadiq has been able to gather support of a group of the Pakistan People’s Party (PPP) led by Dr Safdar Abbasi, Jamiat-e-Ulema Pakistan Noorani (JUP), Jamiat Ulema-i-Islam – Fazl and other clerics in the constituency.
The run-up to the by-poll has seen extensive campaigns from both parties, each trying to stake its claim on the ‘Takht-i-Lahore’.
PML-N leaders including Hamza Shahbaz, Railways Minister Khawaja Saad Rafique, Abid Sher Ali, Marvi Memon, and Sheikh Waqas Akram addressed several corner meetings in the constituency.
While PTI chief Imran Khan along with other party leaders also visited the constituency, addressed corner meetings and inaugurated election offices besides holding public gatherings at Samanabad and Mozang.
