Over 100 reformists, including Mohammad Reza Khatami the brother of former president Khatami, were arrested on Saturday night, leading reformist Mohammad Ali Abtahi told Reuters on Sunday.
"They were taken from their homes last night," said former vice president Abtahi.
He said more arrests were expected.
The arrested reformists were members of Iran's leading reformist party Mosharekat.
Thousands of people clashed with police on Saturday after the disputed election victory of President Mahmoud Ahmadinejad sparked the biggest protests in Tehran since the 1979 Islamic revolution.
Supreme Leader Ayatollah Ali Khamenei told Iranians to respect Ahmadinejad's victory in a presidential election that his closest challenger described as a "dangerous charade."