"Security forces came to his home (in Tehran) at 3 a.m. (2330 GMT) and arrested this political activist," the Jaras website said. It said his niece was detained a few hours earlier.
Yazdi, who heads the banned Freedom Movement, was foreign minister in Iran's first government after the 1979 Islamic revolution that overthrew the U.S.-backed shah, but was sidelined as religious hardliners took over.
Yazdi, who was also detained after Iran's disputed election in June, is an important opposition voice in Iran but has no influence on state policy and limited popular support. (Writing by Fredrik Dahl; editing by Jon Boyle)