The Iranian authorities must release a 76-year-old scholar held incommunicado for 41 days, apparently for his peaceful criticism of the Iranian presidential election, said Amnesty International.
The organization also expressed grave concern for the health of Dr Mohammad Maleki, and said it fears that his life is in danger as he is suffering from serious illnesses.
Mohammad Maleki, a former Chancellor of Tehran University, has been held without charge in Tehran's Evin prison since his arrest in August. His two-month temporary detention order was renewed on 22 October, though he was unable to challenge it and no reasons were given as to why it was extended.
The officials who arrested Mohammad Maleki told his wife Ghodsi Mir Moez that he was being investigated for stirring up unrest and for what they alleged were his links to the People's Mojahedin Organization of Iran, a banned opposition group.
Many of those arrested since the election have faced similar allegations by the authorities.
Dr Maleki's family has stressed that he is not affiliated to any political party and did not vote in the 12 June presidential election in Iran.
He criticized the conduct of the elections but did not publicly express a view about any one of the four candidates that stood for election.
Amnesty International said that it considers him to be a prisoner of conscience, held solely for the peaceful expression of his views.
Before his arrest Mohammad Maleki had been receiving regular treatment for prostate cancer, which he has not been given while in custody.
On 23 October, his wife, Ghodsi Mir Moez, expressed concern in an interview with a German radio station that his health is likely to have deteriorated.
His family and lawyer were allowed to visit him once, on 14 September. Ghodsi Mir Moez said afterwards:
"I noticed the doctor's physical condition had worsened and his hands and feet were trembling constantly and he did not have enough strength to stand on his own feet." She also said he had complained that he was "in severe pain and was suffering from an increased heart rate, low blood pressure, and a burning sensation during urination."
Amnesty International has called on the Iranian authorities to immediately provide Dr Mohammad Maleki with all necessary medical treatment, including that available outside prison if necessary.
The organization has also urged the authorities to clarify his state of health as a matter of urgency and to grant him immediate and regular visits by his family and lawyer.