Photos emerged of the moment paramedics transferred the man into a waiting ambulance on Gorgie Road.
It was later confirmed the man had fallen ill not far from Saughton prison.
One witness said: “It looked pretty bad, he was on a spinal board.”
Police were also scrambled to the scene and were seen escorting the ambulance to the Royal Infirmary.
A Police Scotland spokeswoman said: "At about 1.05pm a male prisoner was taken unwell while travelling in a security van."