Around 7.68 lakh voters are eligible to decide the fate of the candidates in the four assembly segments of Phagwara (reserve), Dakha, Jalalabad and Mukerian.
Polling will be held from 7 am till 6 pm. The results of the elections will be declared on October 24.
Seven companies of paramilitary forces have been deployed in the four assembly segments, officials said, adding that 920 polling stations have been set up. Some polling stations in Dakha and Jalalabad have been categorized as sensitive.
The ruling Congress would like to maintain its winning momentum in the bypolls while the SAD-BJP alliance and AAP are looking at their political revival.
It had registered an emphatic victory in 2019 Lok Sabha polls by winning eight out of total 13 parliamentary seats in the state.
In Phagwara (reserve), the main contest is between Congress candidate and former IAS officer B S Dhaliwal and BJP's Rajesh Bagha. Besides, the AAP, BSP and the Lok Insaaf Party have also fielded their candidates.
The Phagwara seat fell vacant after sitting MLA Som Parkash was elected to the Lok Sabha from Hoshiarpur.
On the Dakha seat, the Congress has fielded Sandeep Sandhu, political secretary of Chief Minister Amarinder Singh. The SAD nominee from the seat is former MLA Manpreet Singh Ayali.
It is being seen as a keen contest between the Congress and Akalis. Chief Minister Singh had himself taken out two -roadshows in favour of Sandhu who has been Singh's trusted lieutenant. The Dakha seat got vacant following the resignation of former AAP leader and noted Supreme Court lawyer H S Phoolka from the state assembly.
Phoolka had resigned in October last year over the Congress government's alleged failure in taking action against those involved in the desecration of religious sects in 2015 in the state.
In Jalalabad, considered an Akali bastion, there is a straight contest between SAD's Raj Singh Dibipura and Congress's Raminder Singh Awla.
The Jalalabad seat fell vacant after sitting MLA and SAD chief Sukhbir Singh Badal was elected to the Lok Sabha from Ferozepur parliamentary segment in the 2019 General Election.
The top leadership of Akali Dal including Sukhbir Badal, union minister Harsimrat Kaur, Bikram Majithia held poll rallies to retain Jalalabad seat, which is considered as Akalis' bastion.
From Mukerian, the Bharatiya Janata Party has fielded Jangi Lal Mahajan.
The Congress nominee is Indu Bala, the wife of the late Congress MLA Rajnish Kumar Babbi, whose death necessitated the byelections.
In the 117-member Punjab Assembly, the Congress at present has 77 legislators. The SAD has 13 MLAs while the strength of AAP legislators is 19.The BJP and Lok Insaaf Party have two MLAs each.