Human Rights Watch accused Egyptian authorities on Wednesday of failing to take serious steps to improve conditions in overcrowded prisons which are causing deaths.
The government denied the accusations.
The New York-based group said it had documented nine deaths in custody since mid-2013, when the army's ouster of Islamist President Mohamed Mursi was followed by a crackdown on his supporters.
Human Rights Watch said authorities "are taking no serious steps" to deal with the issue.
The group said some detainees appeared to have died after torture or physical abuse while "many appear to have died because they were held in severely overcrowded cells or did not receive adequate medical care for serious ailments".
Citing interviews with relatives and lawyers, it called the conditions many detainees faced "life-threatening" and detailed the deaths of five men from beating and lack of medical care.
Interior Ministry spokesman Hany Abdel Latif said "such talk has no basis in truth."