The family of Aziz Majidzadeh, 54, said they were contacted by him who informed them he was being held in the notorious Evin Prison, in the capital Tehran.
Kiaa Aalipour, spokesman for advocacy organisation Article 18, told World Watch Monitor that Majidzadeh has yet to be charged but his interrogations have focused on activities related to his Christian faith.
Aziz was among 20 other Christians in a workshop near the city of Karaj, last month, when security forces raided the premises.
The spokesman said security forces "arrested one of the friends of this group of Christians at his home and forced him to come with them to the workshop. They came in and started filming everything, saying that they were friends of their friend and were shooting footage for a Persian-language Christian satellite channel".
While most of the group were released after being interrogated, Majidzadeh - who has been arrested before for activities related to his faith - was beaten and detained, according to Aalipour.
Iranian Christians Maryam Rostampour and Marziyeh Amirizadeh, who spent eight months in Evin Prison described it as "the most brutal prison in the world".
They wrote about their experiences in a book, 'Captive in Iran', after they were released in 2010 and granted asylum in the US.
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