Zia Nabavi, a spokesman for the Council to Defend the Right to Education, was arrested during the unrest following the June 2009 presidential election and jailed on charges that included "assembly with intent to conspire against national security" and cooperation with the banned exile group, the People's Mujahedin Organization of Iran (MKO).
He is serving a 10-year jail sentence in Karun prison in the southwestern city of Ahvaz.
His letter to Mohammad Javad Ardshir Larijani, the head of the judiciary's human rights department, was published on the Daneshjoonews and Kalame websites on May 4.
Peyman Roshanzamir, a former cell mate of Nabavi in Karun, in an interview with Radio Farda on May 4 described the prison as overcrowded and dirty.
"I was surprised to see that over 1,500 prisoners had been sleeping in the yard for a year, regardless of the severe weather conditions," he said.
He added that they are forced to find covered spots in the yard should it rain at night.
Roshanzamir also said that when it rained, the yard would become a massive puddle of rainwater contaminated with sewage.
"After clearing up the mess within hours, the inmates would spread their blankets on the same ground to sleep once more," he said.
Nabavi in his letter also complained that political prisoners were often held in the same block as drugs offenders.