The Fars news agency identified the small but vocal crowd which gathered on Sunday outside the building where Karroubi lives as "students and families of martyrs" of the Iran-Iraq war.
Pictures carried by the pro-government Borna news agency showed the building had been defaced with red colouring and slogans pronouncing "Death to Karroubi" had been scribbled on the walls.
The walls had also been plastered with death slogans against main opposition leader Mir Hossein Mousavi and former president Mohammad Khatami.
"We want the judiciary to put the leaders of sedition on trial as soon as possible," Fars quoted the protesters as saying. They also denounced Karroubi as a "hypocrite" — the term used by Iranian officials for the enemies of state.
Some brandished placards that read "Karroubi is a Mossad agent" — linking the two-time former parliament speaker to the intelligence service of Iran's archfoe Israel.
A relative of Karroubi confirmed the incident to AFP and said the cleric would soon make a statement.
Karroubi and Mousavi have led a protest movement against President Mahmoud Ahmadinejad after his June re-election, which they reject as massively rigged.
Karroubi was attacked by hardliners during Iran's annual revolution day rally on February 11 and his car was shot at earlier in January in the city of Qazvin west of Tehran.
The outspoken cleric, who with Mousavi ran against Ahmadinejad in the June vote, has infuriated hardliners by charging some post-election detainees had been raped in jail. Iranian authorities vehemently deny the allegations.
