A leading Iranian lawyer and women's rights campaigner, Shadi Sadr, was arrested on her way to Friday prayers in the capital that were attended by scores of vote protesters, her husband said.
"Shadi called me from an unknown location and said she was arrested by plain-clothes officials who forcefully got her into a car," Hossein Nilchian told AFP.
He said Sadr was accompanied by her friends and she was the only one taken away.
Sadr, 34, is a well-known women's rights activist who has campaigned against one of Iran's internationally-condemned practices of death by stoning for adulterers. She has defended several such convicts as a lawyer.
Rights group Amnesty International called for Sadr's immediate release, after what it called her violent arrest.
"Shadi Sadr was walking with a group of women's rights activists along a busy road when unidentified plain-clothed men pulled her into a car," it said in a statement.
"She lost her headscarf and coat in the ensuing struggle but managed briefly to escape. She was quickly recaptured and beaten with batons before being taken away in the car to an unknown location."
Amnesty Middle East and North Africa head Malcolm Smart said: "This was an illegal, arbitrary and violent arrest in which no attempt was made by the authorities to show identification or provide any explanation for their action.
"This is the latest of a continuing series of high profile arrests of Iranians," he said, calling for Sadr "to be immediately and unconditionally released."
Thousands of supporters of opposition leader Mir Hossein Mousavi demonstrated in Tehran streets after the weekly Muslim prayers led by influential cleric Akbar Hashemi Rafsanjani.
Iran has jailed dozens of journalists, political activists and reformist leaders in the wake of President Mahmoud Ahmadinejad's disputed re-election in a June 12 poll.
At least 20 people have died in clashes with security forces and hundreds of opposition protesters have also been detained.
Rafsanjani on Friday called for the release of the detainees.