Iranian police have arrested former Tehran prosecutor Saeed Mortazavi, Iranian news agencies say.
Mr Mortazavi, considered a close aide of President Mahmoud Ahmadinejad, was taken to Tehran's Evin prison, the semi-official Fars agency reported.
Until January, he was head of Iran's wealthy social welfare organisation but was removed under pressure from the Majlis - Iran's parliament.
Mr Mortazavi, 45, was placed under US sanctions in 2010.
He and other officials were accused by Washington of "sustained and severe violations of human rights".
Analysts say his arrest could be the result of disputes between President Ahmadinejad on one hand and parliament and the judiciary on the other.
The arrest was "possibly" in connection with allegations of torture and the deaths of prisoners after the controversial 2009 election, Fars reported.
A few days after he was removed last month as head of the social welfare fund, he was rehired by President Ahmadinejad, but this time as official caretaker of the same organisation, angering some lawmakers.
While defending the former prosecutor in parliament, Mr Ahmadinejad accused the head of parliament and the head of the judiciary of corruption, which analysts say might be one of the reasons behind Mr Mortazavi's arrest.
Mr Ahmadinejad's controversial re-election in 2009 - and the violent suppression of subsequent opposition protests - has widened the rift between conservatives and reformists within Iran's political establishment.
Correspondents say parliament has been hostile to Mr Ahmadinejad for most of his second four-year term which expires in August.
Parliament Speaker Ali Larijani is a likely frontrunner in the race to succeed Mr Ahmadinejad.