Judiciary officials entered Karoubi's office in northern Tehran and told him and others inside to leave, ILNA said.
"Karoubi's office has been sealed off upon the Tehran prosecutor's order," it quoted Esmail Gerami-Moghaddam, a spokesman for Karoubi's party, as saying. Karoubi came fourth in June's disputed presidential election.
There was no immediate comment from Iranian officials.
Website mowjcamp.ir said agents raided and searched the home of Morteza Alviri and took him away. It also confirmed the closure of Karoubi's office.
On Monday security forces raided an office run by allies of opposition leader Mirhossein Mousavi, the election runner-up, and confiscated documents, his website said.
Mousavi's website said it was 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.
Karoubi, whose newspaper was closed down three weeks ago, angered hardliners 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.
Supreme Leader Ayatollah Ali Khamenei ordered the closure of Tehran's Kahrizak detention centre in July after reports of abuse there and the semi-official Mehr News Agency on Monday said a trial of people involved would start in coming days.
Karoubi 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.
Ahmadinejad has suggested his opponents were behind any such incidents, saying they had "infiltrated" government forces.
The hardline president shored up his position last week when parliament approved most of his new government ministers, after almost three months of political turmoil in the world's No.5 crude exporter.
POLICE WARNING
The election and its turbulent aftermath plunged Iran into its deepest internal crisis since the 1979 Islamic revolution, exposing deepening divisions within its ruling elites and adding to tension with the West.
Reformist leaders say it was rigged to secure Ahmadinejad's re-election. He denies the charge, describing it as the healthiest vote ever held in the Islamic state.
The protests were put down by the elite Revolutionary Guards and a pro-government Islamic militia, but pro-reform leaders have made clear they will continue their fight over the poll.
The opposition says more than 70 people were killed in the unrest. The authorities estimate the death toll at 26 people and say members of the Islamic Basij militia were among them.
Hardliners have portrayed the opposition protests as a foreign-backed bid to undermine the Islamic Republic.
Iran's police chief Esmail Ahmadi-Moghaddam warned against using annual demonstrations in favour of the Palestinians on Sept. 18 to stage new election-related rallies.
"Certainly, events similar to what have happened in June will not be repeated," the official IRNA news agency quoted him as saying late on Monday.
But Karoubi said he would attend and urged others to do the same. "I invite people to participate to support the oppressed Palestinian people," he said on Tuesday.
Mousavi and reformist former President Mohammad Khatami have also issued defiant statements over the last few days, after the the Guards accused them of trying to topple the Islamic system.
Rights groups say thousands of people, including senior pro-reform figures, were arrested after the presidential poll almost three months ago. Most of them have been freed but more than 200 remain in jail, according to the opposition.
A member of parliament's investigative committee said those who did not play a major part in the unrest should be immediately released, IRNA reported.
MP Farhad Tajari also expressed criticism about the conduct of mass trials which started last month of more than 100 reformers, activists and others accused of orchestrating the protests as part of a "velvet revolution" bid.
Khatami last month said confessions made by detainees at the trials were obtained under "extraordinary conditions" and were invalid. Those in the dock include several of his allies. (Editing by Louise Ireland)
