Mr Mohammad Asghar was arrested in 2010 after he claimed to be Prophet Mohammed in letters sent to various officials. A judge convicted and sentenced him last Thursday following a trial.
But a lawyer who defended Mr Asghar said he suffers from mental illness and the case was really a property dispute.
Amnesty International's deputy Asia Pacific director Polly Truscott said:
'Mohammad Asghar is now facing the gallows simply for writing a series of letters. He does not deserve punishment. No one should be charged on the basis of this sort of conduct. The blasphemy laws undermine the rule of law, and people facing charges risk death and other harm in detention.
Pakistan must immediately release Mohammad Ashgar and reform its blasphemy laws to ensure that this will not happen again.'Reports say people in the country, accused of blasphemy have been attacked and killed by angry vigilante mobs.
Two prominent politicians who criticised the blasphemy law were murdered in recent years. One of the politicians was shot by his own bodyguard, who then attracted adoring crowds.