The son of Iran's former president Akbar Hashemi Rafsanjani was arrested after he returned from abroad on Monday, two days after authorities jailed Rafsanjani's daughter, the official IRNA news agency reported.
Mehdi Hashemi was apprehended after an hours-long interrogation by prosecutors after his arrival in Tehran from Dubai after a flight from Britain, where he had lived for the past three years.
Iran had issued an arrest warrant for him in 2010.
On Saturday, Hashemi's sister Faezeh Hashemi was arrested and imprisoned to serve a six-month jail term for "propaganda against the regime." The sentence had been delivered early this year but not imposed until now.
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Both Mehdi and Faezeh Hashemi were accused by authorities of involvement in 2009 street protests that erupted after President Mahmoud Ahmadinejad was given a second term in elections that the opposition said were marked by vote fraud.
Faezeh Hashemi was briefly arrested with thousands of others after those demonstrations, while Mehdi Hashemi fled the country.
Their father is an influential cleric who served as president from 1989 to 1997 and who is seen as a moderate voice isolated by hardliners in Iran's regime.
Rafsanjani, who still heads the country's top political arbitration body, has faced demands from conservatives that he publicly condemn opposition leaders Mir Hossein Mousavi and Mehdi Karroubi, both of whom languish under house arrest.
Ahmadinejad, who beat Rafsanjani to secure the presidency in 2005, is to step down next year, having served his maximum two terms. Iran's presidential election to choose a successor is due to be held on June 14, 2013.