Usman Masih, 29, died from a heart attack but an autopsy report revealed his body was covered in bruises and welts.
The British Pakistani Christian Association (BPCA) said the state of the body of the married father of two children created "suspicion".
Wilson Chowdhry, Chairman of the BPCA, said: "Negligence and violence has led to the death of a man believed to be totally innocent by the Christian community.
"The lesions on the victim corpses suggests foul play and a brutal ending.
"Pakistan has ratified the UN Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, yet they persist with the most undignified and brutal incarceration of prisoners."
Mr Masih was arrested with over 500 men after a large group of Christian's sorrow turned to deadly rage, resulting in two Muslim men being killed in Lahore.
The Christian community of Youhanabad had been protesting the suicide bombing attack on two churches , which killed 15 people and injured 78 in Lahore as extremists continued to target Christians in the country.
BPCA said in a statement that all the arrested men were tortured and detained for over a year and a half before the majority were released without charge.
However, the arrests led to 83 men being charged of which 42 were later convicted with murder offences a few months later.
The Christian charity said legal teams fighting the case believed that over 30 of those convicted were blameless.
Mr Chowdhry added: "That should be of no surprise to us however, as news of the murder of the two Muslims dominated Pakistani media causing huge national offence for the outrageous audacity of Christians daring to kill Muslims.
"The 15 Christians killed and 80 Injured in the twin church attack became of no consequence.
"Christian lives are clearly of no value in a nation that sees them as a pariah community."
