Egypt is holding political prisoners in "prolonged and indefinite solitary confinement" that amounts to "torture," an international rights group said Monday.

In a new report, Amnesty International said dozens of detained human rights activists, journalists and members of the opposition held in solitary confinement face "horrendous physical abuse." Such treatment results in "panic attacks, paranoia, hypersensitivity to stimuli, and difficulties with concentration and memory," it said.

Egypt has detained thousands of people, mainly Islamists but also several prominent secular activists, since the military overthrew an elected but divisive Islamist president, Mohammed Morsi, in 2013. Morsi himself has reportedly been held in solitary confinement for most of the last five years.

"Under international law, solitary confinement may only be used as a disciplinary measure of last resort, but the Egyptian authorities are using it as a horrifying 'extra' punishment for political prisoners," said Najia Bounaim, Amnesty's North Africa campaigns director.

The London-based group said it has documented 36 cases of prisoners being held in solitary confinement, including six who have been isolated from the outside world since 2013. It said the prisoners in solitary confinement remain in their cells for 22 hours a day.

Amnesty and other rights groups say torture and abuse are widespread in Egyptian prisons.

Basing its report on dozens of interviews with former prisoners and with family members of current prisoners, the group said abuses range from extended beatings to lack of food, humiliation and restricted movement for years on end.

Prisoners held in solitary confinement "suffer depression, insomnia and an unwillingness to socialize or speak to other people when released back into the prison population," it said.

The prolonged solitary confinement is usually aimed at extracting confessions and punishing prisoners for protesting ill-treatment, but some are held in solitary confinement purely because of their past political activism, Amnesty said.

"Prison conditions in Egypt have always been bad, but the deliberate cruelty of this treatment shows the wider contempt for human rights and dignity by the Egyptian authorities," Bounaim said.

Egypt's Interior Ministry in the past has denied allegations of systemic torture, blaming any abuses on individuals and saying they are held accountable. Several officers have been tried and convicted of torture, while others have been acquitted.

Egypt has said enhanced security measures are needed to combat the Islamic State extremist movement and other armed groups that have stepped up attacks since 2013.