Morteza Alvrir, an aide to Karroubi, was also arrested and his property searched, according to the website mowjcamp.ir. It confirmed the closure of Karroubi's office.
Karroubi, whose newspaper was closed down three weeks ago, angered members of the government last month by saying some imprisoned protesters were raped and abused in jail.
The authorities have rejected the allegation as baseless but the judiciary and parliament have agreed to look into the issue.
Karroubi said on his party's website this week he had handed over films and other material about abuse of three detainees to a special investigative parliamentary committee.
Iranian security forces raided on Monday an office run by allies of opposition leader Mir Hossein Mousavi, the election runner-up, and confiscated documents, according to his website.
Mousavi's website said his offices were the premises of a committee set up by him to look into post-election events, including the number of dead and the treatment of people detained during the huge opposition demonstrations that followed the vote.
Ayatollah Ali Khamenei, Iran's supreme leader, ordered the closure of Tehran's Kahrizak detention centre in July after reports of abuse there and the semi-official Mehr News Agency said a trial of people involved would start in coming days.
Ahmadinejad has suggested his opponents were behind any such incidents, saying they had "infiltrated" government forces.
Parliament approved most of his new government ministers after almost three months of political turmoil in the world's No.5 crude exporter.
The election and its turbulent aftermath plunged Iran, the world's No 5 crude exporter, into its deepest internal crisis since the 1979 Islamic revolution, exposing deepening divisions within its ruling elites and adding to tension with the West.
The protests were put down by the elite Revolutionary Guards and a pro-government militia, but pro-reform leaders have made clear they will continue their fight over the poll.