Mushtaq suffered a heart attack inside his cell at Kashimpur High Security Prison on Thursday evening, it was learnt. He was transported to Shaheed Tajuddin Medical College Hospital, but there he was declared dead on arrival, said Senior Jail Superintendent Md Gias Uddin said.
The 53-year-old writer was behind bars since May last year following his arrest in a case started by Rapid Action Battalion (RAB).
On August 20 last year, he was shifted to the Kashimpur prison, Gias Uddin said.
Md Sharif, a doctor at Shaheed Tajuddin Medical College Hospital's emergency department, said "Mushtaq was brought dead at the hospital. However, the exact cause of his death can be confirmed once the autopsy is done."
"Mushtaq's body will be handed over to his family on Friday after the completion of the autopsy," Gias Uddin also added.
Earlier, RAB arrested Mushtaq, who was also a businessman, and cartoonist Ahmed Kabir Kishore last year on charges of spreading disinformation against the government on social media.
Calls for Mushtaq's release were widespread and sustained throughout his time in prison. Soon after his arrest, more than 300 dignitaries in Bangladesh issued a joint statement demanding his release, alongside cartoonist Ahmed Kishore.
Amnesty International reiterated the demand in a statement as recently as January, saying the duo (Kishore and Mushtaq) had been arrested "solely for the peaceful exercise of their right to freedom of expression."