"Yesterday Mohammad Ali Dadkhah was arrested with a number of other lawyers at his office," said fellow lawyer Mohammad Seifzadeh, who is also a founding member of Ebadi's Human Rights Defenders Centre.
The group is an outspoken critic of the human rights situation in Iran and has faced mounting pressure since December 2008 after its office was shut down in a police raid.
Seifzadeh said Dadkhah, a prominent human rights lawyer, had been discussing on Wednesday a new code introduced by the judiciary on the workings of Iran's bar association but which has yet to come into force.
"The new code damages lawyers' independence and it is against the law," Seifzadeh said.
Another member of the centre, Abdolfatah Soltani, has also been detained along with scores of reformist leaders and journalists in the wake of Ahmadinejad's re-election, which triggered massive street protests.
Opposition leader and main presidential challenger Mir Hossein Mousavi has dismissed the election as a "shameful fraud" and branded the next government "illegitimate."
