Qamar David was serving a life sentence in Karachi's central jail for insulting the Koran and the Prophet Muhammad.
Officials say he died of a heart attack but his family say that as a fit and healthy man, this is unlikely be true. Pakistan's penal code prescribes punishments up to death for blasphemy against Islam and the Prophet.
The law has elicited strong criticism - although recently liberals are more nervous about speaking out against it. [BBC] Read more
