The deceased was identified as Ujjal Pramanik, 35, son of Abdul Aziz of Lakshmipur village in the upazila.
Witnesses said Awami League supporters brought out a procession around 7 pm after the vote counting in Paharpur-Lakshmipur School polling center as their candidate bagged large number of votes in the upazila parishad elections.
When the procession reached Haziwahedmor area of the village, Jasad general secretary Habibur Rahman of Chitholia Union unit hurled a heavy metal object at the procession which hit Ujjal leaving him severely injured.
Later, doctors declared him dead after he was taken to the Upazila Health Complex.
Officer-in-Charge of Mirpur Police Station Abul Kalam said the cause of the death will be clear after getting the post-mortem report of the deceased.
In the 3rd phase, elections to 117 upazila parishads across the country ended on Sunday evening amid sporadic violence and allegations of vote rigging and other irregularities.
The first phase and second phase elections were held on March 10 and 18 respectively, while the fourth phase polls are scheduled to be held on March 31.