An Iranian political prisoner, who was arrested during mass protests in December 2017, has died due to the injection of an unknown substance in prison.

Mehdi Salehi, whose death was announced in social media on Thursday, had a stroke and went into a coma in January because of the injection of an unknown drug by Esfahan prison officials, human rights monitors said at the time.

A source close to Salehi's family told Iran International that he was chained to the bed in hospital. Recently, he had come out of coma but suddenly the family was told that he passed away.

He was among five protesters sentenced to death, with Iran Human Rights Organization warning about the possibility of authorities secretly executing them.

Large nationwide protests in December 2017 were triggered by rising prices and turned into anti-Islamic-Republic unrest in over 100 cities and towns. Hundreds of striking and protesting workers and labor activists have also been arrested since 2017, many spending months in prison. Some are still detained without trial.

In a report released this week, Amnesty International slammed Iran for prisoner deaths resulting from deliberate denial of medical care, turning prisons into "waiting rooms for death".

The global rights organization has documented how prison authorities routinely cause or contribute to deaths in custody, including by blocking or delaying prisoners’ access to emergency hospitalization.

Sixty-four of all these prisoners died inside their prison cells meaning they were not given even basic medical supervision in their final hours.