Shamsuzzaman Dablu, general secretary of Jibannagar Municipal BNP, died shortly after being detained during a joint forces operation in Chuadanga.
Family members and party leaders have alleged that he died due to torture.
The army, however, claimed that Dablu suddenly fell ill during interrogation and later died from a stroke.
A foreign pistol and four rounds of ammunition were reportedly recovered from him during the operation, which the army described as part of a routine effort to recover arms.
The incident occurred around 10:30pm on Monday, when a joint army team carried out an operation in the Jibannagar municipal area. Dablu was detained from his business, Hafiza Pharmacy, located in front of the health complex, and later taken to the hospital.
Following news of his death, BNP activists gathered outside Jibannagar Upazila Health Complex on Tuesday morning.
They blocked roads by placing tree logs and set fire on the street while chanting slogans, calling his death a murder and demanding punishment for those responsible.
Police, in the presence of an executive magistrate, conducted an inquest on Dablu’s body at the health complex. While his family has demanded that a case be filed, authorities said no case has yet been registered.
Mahmud Hasan Khan, president of Chuadanga district BNP and candidate for Chuadanga-2, said: “BNP leaders and activists are being tortured under the pretext of operations. The intensity of the torture was so severe that Shamsuzzaman could not survive.”
Deputy Commissioner Mohammad Kamal Hossain urged protesters to remain calm and said that if the incident is proven to be a killing, legal action would be taken.
Chuadanga district Superintendent of Police Mohammad Monirul Islam said that Dablu was taken to the hospital after falling ill, where doctors declared him dead. “Lieutenant Colonel Imran of the army informed us of the details of the incident,” he said.
The postmortem was conducted on Tuesday morning at the site by Jibannagar Upazila Executive Officer Md Al Amin, in the presence of police.