Iranian authorities have detained Hossein Fereydoun, brother of president Hassan Rouhani, for financial corruption, the judiciary spokesman said on Sunday.
Some Rouhani supporters have interpreted the charges against Fereydoun as a move by the judiciary to discredit the president.
State television quoted spokesman Gholamhossein Mohseni Ejei as saying Fereydoun was summoned for questioning on Saturday in a case in which he faces unspecified financial charges.
Ferydoun, who is also a close adviser to Rouhani, was later detained.
"If Fereydoun arranges bail, he will be released," Mohseni Ejei was quoted as saying.
"He has been heard several times, and others have also been questioned, and some of them are being held ... and the case continues."
No details were available on the amount of bail he needs to pay.
Fereydoun has served as a senior diplomat who took part in the talks that led to a 2015 nuclear deal between Iran and world powers.