“Security forces came to his home (in Teheran) at 3am (2330 GMT) and arrested this political activist,” the Jaras website said. It said his niece was detained a few hours earlier.
Mr Yazdi, who heads the banned Freedom Movement, was foreign minister in Iran’s first government after the 1979 Islamic revolution that overthrew the US-backed shah, but was sidelined as religious hardliners took over.
Mr 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.
Mr Yazdi’s arrest came as Iranian television reported that the death toll from the protests had risen to 15.
Earlier, another opposition figure criticised Iran’s hardline rulers for killing innocent people during a religious festival.
The clashes were the most violent since a contested June 12 presidential vote sparked political turmoil across the Islamic Republic.
“What has happened to this religious system that it orders the killing of innocent people during the holy day of Ashura?”, moderate cleric Mehdi Karoubi, who came fourth in the election, said in a statement, the Jaras website reported.
“Why is such a holy day not respected by the rulers?”
Opposition websites said police opened fire on protesters in central Tehran.
Fifteen people were killed in the capital and other Iranian cities when tens of thousands of opposition backers took to … >>>