Iranian activist Farideh Moradkhani. (Photo courtesy of official Twitter account)
Farideh Moradkhani comes from a branch of the family that has a record of opposition to Iran's clerical leadership and has herself been jailed previously in the country.
Her brother Mahmoud Moradkhani wrote on Twitter that she was arrested on Wednesday after going to the office of the prosecutor following a summons.
Then on Saturday, her brother posted a video on YouTube, with the link shared on Twitter, where she condemned the "clear and obvious oppression" Iranians have been subjected to, and criticized the international community's inaction.
"Free people, be with us! Tell your governments to stop supporting this murderous and child-killing regime," she said.
"This regime is not loyal to any of its religious principles and does not know any law or rule except force and maintaining its power in any way possible."
It was not clear when the video had been recorded.
She complained that the sanctions imposed against the regime over its crackdown were "laughable" and said Iranians had been left "alone" in their fight for freedom.
Moradkhani is the daughter of Khamenei's sister Badri who fell out with her family in the 1980s and fled to Iraq at the peak of the war with Iran's neighbour. She joined her husband, the dissident cleric Ali Tehrani who was born Ali Moradkhani Arangeh.
She has gained prominence as an anti-death penalty activist and was last arrested in January this year.
That arrest came after an October 2021 video conference in which she lavishly praised Farah Diba, the widow of shah Mohammad Reza Pahlavi who was ousted by the 1979 Islamic Revolution.
Human Rights Activists News Agency (HRANA) said she had then been released on bail in April and her arrest last Wednesday was to begin serving an existing 15-year sentence.
The charges were not immediately clear.
According to the United Nations, High Commissioner for Human Rights around 14,000 people have been arrested over the protests that began after the death of Mahsa Amini who had been arrested by the morality police.
At least 416 people including 51 children have been killed in the crackdown, according to Norway-based Iran Human Rights (IHR).