Abbas Amir-Entezam, regarded as a liberal, had spent decades in jail after being found guilty of espionage and treason shortly after the 1979 Islamic revolution.
The Fars news agency said he had been at home at the time and could not be resuscitated. It was not clear how long he had been out of prison and under what conditions he had been allowed to return home.
Amir-Entezam was widely considered by human rights groups to be Iran's longest-serving political prisoner. He was a deputy prime minister and government spokesman in the provisional government headed by Mehdi Bazargan after the revolution that overthrew Iran's Shah. But he opposed moves to turn the state into an Islamic republic.
After serving a 17-year sentence, he was arrested again in 1998 after making critical statements about the former head of the Evin prison near Tehran. After a brief period of liberty he was detained again in the early 2000s and sent back to prison after calling for a referendum on the country's political system.
In 2017 he gave an interview to the Tarikh Online ("History Online") site. He was unable to hold back his tears as he recalled being prevented from seeing his family for the first "six or seven years" of his detention.
He said his jailers had forbidden him from wearing shoes, even confiscating those he had made himself. A number of human rights organisations expressed their support for Amir-Entezam, and in 1997 he was awarded the Austrian Bruno Kreisky prize for human rights.
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