Mohammad Olyaeifard was reportedly sentenced to one year in prison in February after speaking out against the execution of one of his clients during interviews with international media.
Olyaeifard said before his arrest that he had been convicted because of an interview he gave to Voice of America's Persian service shortly after his client, juvenile offender Behnoud Shojaee, was hanged for a murder he committed when he was 17 years old.
Olyaeifard is among the very few lawyers in Iran who represent child offenders on death row in the Islamic republic.
Iran is one of the only countries that execute offenders for crimes they committed when they were younger than 18 years old.
This video about the plight of child offenders in Iran was co-produced by Olyaeifard's colleague, Mohammad Mostafaei, who is Iran's leading advocate against child execution.
Amnesty International says Olyaeifard's arrest sends "a chilling message to lawyers in Iran that if they dare to denounce abuses or miscarriages of justice they will face reprisals."
In recent years a number of lawyers have been arrested in Iran over their work and their defense of political prisoners. They include Iranian Nobel Peace Prize winner Shirin Ebadi and two of her colleagues, Abdolfatah Soltani and Mohammad Ali Dadkhah. Mostafaei was also arrested briefly last year.
The work license of at least two lawyers representing the case of Gonabadi dervishes, who've come under pressure by the authorities, have reportedly been canceled by the judiciary.
One of them, Mostafa Daneshjou, told RFE/RL's Radio Farda that the situation puts the independence of lawyers at serious risk. He has called on his colleagues to protest against the pressure on lawyers.
"If not, today was my turn, tomorrow it will be yours," Mostafaei said.
-- Golnaz Esfandiari