Hossein Fereydoun, who was a close adviser to the president, entered Tehran's Evin prison on Wednesday, his lawyer told local media.

He was convicted of "receiving bribes" in May and sentenced to seven years.

An appeals court later reduced the jail term, but ordered him to return any property that he had acquired illicitly and fined him 310bn riyals.

That sum is equivalent to $9m (£7m) on the country's official exchange rate, but $27m (£21m) at the free-market exchange rate.

Fereydoun denied any wrongdoing and some supporters of the president, who is a moderate, said the case was a move by hardliners in the judiciary to discredit him.

The judiciary has said it has no political motivation for the cases it tries.

Mr Rouhani promised to curb corruption in the 2013 and 2017 president election campaigns. However, he has struggled to do so.

The president was born Hassan Fereydoun but changed his surname to a word meaning "spiritual" or "cleric" decades ago after joining a seminary.

In a separate development on Wednesday, the French foreign ministry said the Iranian authorities had detained the French academic Roland Marchal.

The ministry added that it had asked Iranian officials to bring the "unacceptable" situation to an end without delay.

Mr Marchal, a sociologist who focuses on civil wars in Africa, is reported to have been arrested in June along with a French-Iranian colleague at Sciences Po university in Paris, Fariba Adelkhah. Her detention was confirmed in July.