Karroubi was in the town to attend a mourning ceremony for slain opposition protesters organised by a reformist former member of parliament, Sahamnews.org said.
"Around 500 basiji (members of the Basij Islamic militia) and residents of nearby villages surrounded the place where he was and attacked the building with stones, breaking windows," it said.
After four hours, anti-riot police finally intervened to get Karroubi out of the building.
"As his car was pulling away, it was attacked and hit by gunfire. But, as it is an armoured car, only the windows were damaged."
There was no immediate word from the authorities on the incident.
