The man has been identified as 50-year-old Md Mohibullah, a member of Awami League's Kodala Union unit in Rangunia Upazila.
He died while undergoing treatment at a private hospital in the port city on Tuesday night, according to police.
“The Hifazat activists attacked Mohibullah, Abdur Jabbar and Dildar Azam Liton. Mohibullah was taken to a hospital after he suffered severe head injuries."
Mohibullah, Jabbar and Liton were heading to a local market in Kodala Union by rickshaw when they came under attack. Local BNP and Jamaat-e-Islami activists have links to the assaults, Rangunia Police Station OC Mahbub Milki said.
Jabbar and police started two cases over the attack with Rangunia Police Station against 64 people and 150 unidentified individuals.
At least four people were arrested over the incident, two of them are accused in the cases, ASP Shamim said.
Supporters of Mamunul came to his rescue after he was detained at Royal Resort in Narayanganj’s Sonargaon with a woman who he claims is his second wife.
The supporters, angered by the confinement of the Islamist leader, pelted the resort with brickbats, vandalised cars and burnt tyres blocking the Dhaka-Chattogram highway, witnesses said.