Mehdi Karroubi's "office was sealed by order of the Tehran prosecutor after a number of agents with a prosecutor's warrant came to his office near the Saadabat Palace in northern Tehran," ILNA news agency quoted Esmaeel Gherami Moghaddam, spokesman of Karroubi's political party, as saying.
"They searched the premises and collected documents, CDs, computers and films," he said. "They also arrested the chief editor of the Etemad Melli website."
The website is run by Karroubi's party, which goes by the same name.
Karroubi, who was defeated by incumbent Mahmoud Ahmadinejad in the June 12 presidential election, has been a vocal opponent of the re-elected president.
He and Ahmadinejad's main electoral rival, Mir Hossein Mousavi, have refused to acknowledge the outcome, saying it was the result of massive vote rigging.
In the aftermath of the election, supporters of the two defeated candidates poured on to Tehran streets opposing Ahmadinejad's victory, unleashing the Islamic republic's worst internal crisis in its 30-year history.
The protests turned violent and about 30 people were killed, Iranian officials say, but opposition groups claim 72 people died.
In the immediate aftermath of the unrest, 4,000 people were detained by security forces. Most of them have now been released, but around 140 have been put on mass trials.
The Mehr news agency quoted an unnamed Iranian lawmaker as saying Tehran's recently appointed prosecutor, Abbas Jafari Dolatabadi, wants to hold mass trials of protesters behind closed doors.
Currently, the trials are open to local Iranian news agencies only.
The MP also said Jafari Dolatabadi wants to put on trial those who were responsible for detaining protesters in the notorious Kahrizak prison, located south of Tehran.
Kahrizak was ordered closed by Iran's supreme leader, Ayatollah Ali Khamenei, amid opposition claims that some protesters held there had been physically abused or even beaten to death.
In the past few weeks, Karroubi has been alleging that several protesters, male and female, had been raped in custody. Officials have dismissed these allegations.
On August 17, following those allegations, the judiciary banned the party newspaper, which also carries its name.
Meanwhile, Iran's police chief warned earlier on Tuesday against using the annual pro-Palestinian Quds Day march to stage anti-Ahmadinejad protests.
Karroubi had vowed that his supporters will come out in force on the day, and Mousavi has also called for continued civil disobedience.
The event, first held in Iran but now marked in other places as well, is to express solidarity with the Palestinian people. This year it will be held on September 18.
