In a letter written by Dr Salah Sultan, the former secretary-general in the Higher Council for Islamic Affairs, from his prison cell to his son Dr Mahmoud Salah, he outlines the horrific treatment he and his fellow inmates are subjected to.

In opening, Dr Sultan affirms that spirits are "sky-high" and that the coup authorities will never erode the prisoners' steadfastness or undermine their fortitude. "We told them during interrogation to consider us martyrs," he said.

Dr Sultan writes, describing the prison conditions: "We are now in the high-security Aqrab prison, where conditions are extremely bad. There are 21 of us, including Khayrat El-Shater, Dr Esam El-Haddad, the President's Advisor, and his son Gehad.

"We are facing a slow and systematic death, where we cannot tell day from night due to sunlight depravation and there are no times allocated for walking outside the cells. The cells themselves are essentially sealed tin cans where doors do not open throughout the hours of the day and the air is suffocating.

"Due to the extremely cold weather, the cell is more like a closed freezer, where even water bottles are on the verge of freezing making it impossible to wash or perform ablutions. Our daily food consists of approximately three spoonfuls of rice, a slice of potato or a little bit of lentils and nothing else throughout the day.

"We receive nothing from the outside world; neither family visits nor our requests of food, medicine and treatment, despite a number of inmates suffering from chronic and serious illnesses. We only have one change of clothes which hardly keeps us warm and neither do we have covers or mattresses; just one blanket that functions as a mattress and a cover.

"What we ask from the lawyers is:

• Apply for an official permit to visit issued by the Public Prosecutors Office and to bring in food that has a long shelf life, medicine, covers and mattresses.
• File charges against the Interior Minister for torture, systematic murder and abuse.

"This matter must be raised with all possible media outlets and our plight publicised worldwide."