Mr Karroubi is a leading figure in the opposition Green Movement
Iran opposition leader Mehdi Karroubi's car has been fired on in the northern city of Qazvin, his website reports.
Mr Karroubi, who was in the city for a religious ceremony, was in the car at the time but was not said to be hurt.
The incident happened shortly after he left a house that had been surrounded by what the site called a well-organised group throwing stones.
Mr Karroubi was a reformist candidate in June's election, and has protested strongly against alleged voting fraud.
Along with the main opposition candidate, Mir Hossein Mousavi, he has organised a series of mass protests in the months since.
They have grown into the biggest challenge to the government since the 1979 Islamic revolution.
The semi-official Fars news agency, apparently reporting the same incident, said that Mr Karroubi was forced to leave Qazvin on Thursday after a group staged a rally outside the place he was staying.
The report said the protesters chanted, "God damn Karroubi" and other insults and demanded he leave town.
According to his website, Sahamnews.org, Mr Karroubi was visiting the town to attend a mourning ceremony for opposition supporters killed in protests.
"Around 500 basiji [militia] and residents of nearby villages surrounded the place where he was and attacked the building with stones, breaking windows," it said.
After several hours Mr Karroubi was able to leave with the help of police, but his car was attacked with gunfire as it pulled away.
As it was an armoured car, only the windows were damaged, the site said.