Identifying themselves as members of detective branch (DB) of police, the Jubo League men took Zia to the Amghata area near the house of UP chairman Ripon and shot him in his legs and stabbed him, Babul Sarkar said, adding that later they left him in a critical condition there.
Hearing screams, locals rushed to the spot and took him to the General Hospital.
Shibal Basak, a physician at the hospital, said they found two bullets in the legs and marks of injuries from sharp weapons on his body.
Zia Sarkar, a brick and sand businessman by profession, was referred to DMCH for better treatment, the physician said.
Tarekuzzaman, officer-in-charge (OC) of Sadar police station, said drives are on to nab the people involved with the incident.
Sadar circle additional superintendent of police (ASP) Khandaker Khairul Islam said they primarily came to know that some people identifying themselves as DB picked up Zia and beat him.
Locals said there had been a longstanding dispute between Ripon and former union parishad chairman Mohsina Haque Kolpona over controlling the area.
Babul said his brother was attacked by the followers of Ripon as he is a supporter of Kolpona.