In a statement, he demanded formation of a judicial probe body to transparently look into the death of the writer in jail.
"Mushtaq Ahmed was killed thorough immense torture in jail. Mushtaq was not a looter or a smuggler, terrorist and a robber. He was a talented student of Faujdarhat Cadet College," he said.
The BNP leader also said Mushtaq met a premature death only for his efforts to ensure freedom of thought through social media. "We strongly condemn and protest the incident."
He said the scope for expressing opinions and explanations about various inconsistencies, irregularities and misdeeds of the state and society through the social media is now recognised by all in the democratic world at the current age of information technology.
"But the current authoritarian Awami League government of Bangladesh is not tolerating in any way any kind of criticism against its misdeeds and terrible misrule or any comment and post on the socail media and Facebook," Fakhrul said.
Also read: Writer Mushtaq, arrested under DSA, dies in custody
He alleged that the government is trying to hide its criticisms through various 'black laws'.
The BNP leader also alleged that those who try to express their opinions freely in social media are facing dire consequences as they are either being subjected to enforced disappearance or custodial death. Mushtaq Ahmed was the latest victim of such repressive act."
He hoped that the young generation will wake up through this self-sacrifice of Mushtaq and the freedom of expression, civil rights, good governance, and the rule of law will be 'restored' in the country.
Stating that Mushtaq was a brave and honest man, Fakhrul said he will always remain as a beacon of inspiration to people who are deprived of rights.
He said a 'suffocating' situation has been prevailing in the country for lack of the rule of law and justice.
The BNP leader said the entire nation is going through a tough time while people have no security of their lives and property.
Also read: Writer Mushtaq's body handed over to family after autopsy
Fakhrul said people now cannot raise their voice freely as a new one-party Baksal system has been established in the country.
"Enforced disappearance, killing, crossfire and death in police custody have been made part of activities of the state to silence people," he observed.
The BNP leader also alleged that Bangladesh is gradually turning into a "mafia state" as the government never hesitates to shed the blood of people to secure its power.
He said the death of a nonpolitical and innocent writer like Mushtaq is not natural at all.
The BNP leader prayed for the salvation of the departed soul of the writer and conveyed his profound sympathy to the bereaved family members.
He expressed deep concern as the government still did not release cartoonist Ahmed Kabir Kishore who was arrested along with Mushtaq in May last year.
Also read: Writer Mushtaq's death in custody triggers protests
He demanded the government release Kishore immediately from jail.
Mushtaq who had been arrested in May in a case filed under then Digital Security Act died in police custody around the 8:30pm on Thursday.
Mushtaq suffered a heart attack inside his cell at Kashimpur High Security Prison on Thursday evening. He was taken to Shaheed Tajuddin Medical College Hospital where he was declared brought dead, said Senior Jail Superintendent Md Gias Uddin.