The head of Iran's main reformist coalition, Azar Mansouri, is among at least five prominent opposition figures reported to have been arrested in recent days.
The move represents a widening of the regime's crackdown on dissent in response to the mass anti-government protests in January.
Iranian human rights groups have said they have confirmed the killing of more than 6,000 protesters when security forces brutally put down the demonstrations - and they have suggested that the final figure could be much higher.
Mansouri had called for the truth of what happened not to be covered up.
In a statement last week, she said: "We will not allow the blood of these dear ones to be consigned to oblivion or the truth to be lost in the dust."
