Mostafa Daneshju was detained in Karaj on November 20, but the reason for his arrest has not been made clear, nor is it known where Daneshju is being held.
A fellow lawyer, Farshid Yadollahi, told Radio Farda that Daneshju had previously been sentenced to a prison term and lashes on charges of spreading lies and insulting officials. He reportedly appealed against the sentence.
Yadollahi also said Daneshju has been active in defending the rights of the Gonabadi order of dervishes. "Following the destruction of the Gonabadi worship house in Ghom, [Daneshju] launched a complaint about the legal and religious violations that happened there," Yadollahi said. "He also represented the case of a Sufi who had been attacked in Loristan Province."
Daneshju's arrest comes about 10 days after clashes broke out between dervishes and Shi'ite hard-liners in the western city of Borujerd.
Dozens of people were injured and arrested during the skirmishes, and the Sufis' prayer house was partly destroyed.
Human rights groups have expressed concern over what appears to be increased state pressure on the Sufi order in Iran.
The U.S. State Department says respect for religious freedom in Iran is extremely poor and has been deteriorating since Ahmadinejad came to power in 2005 -- especially for Sufi Muslims and members of the Baha'i faith.