Iranian opposition leaders Mohammad Khatami and Mehdi Karroubi were attacked in their cars during marches marking the Islamic revolution on Thursday, an opposition website and Karroubi's son said.
The website, Rahesabz, also reported that ex-president Khatami's brother Mohammad Reza and his wife were arrested by security forces.
Karroubi's son Hossein told Agence France-Presse that his father was "not injured but his guards who were accompanying him were".
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"They (police and plainclothes men) fired tear gas and were brandishing knives when they clashed with our supporters" before the cleric reached western Tehran's Sadeghieh square from where he was supposed to join the marches.
Karroubi's other son Ali was also arrested, Hossein said.
The reports could not independently be confirmed as the foreign media has been barred from covering the street marches.
Meanwhile there have been reports that security forces have fired paint balls to disperse protesters.
Riot police fired the paint-filled balls after protesters began to chant opposition slogans in Sadeqieh Square, witnesses said.
The square is about a kilometre from a huge pro-government gathering where President Mahmoud Ahmadinejad was delivering a speech on Thursday.
Witnesses said there were no apparent injuries among the several hundred protesters.
Iranian authorities have deployed a massive security force to try to crush expected anti-government rallies on the most important day of the Iranian political calendar.
