Defeated Iranian presidential candidate Mir Hossein Mousavi has urged his supporters not to be provoked, hours after the arrest of one of his senior aides.
In a statement Wednesday on his Web site Kaleme.com, Mr. Mousavi said the arrest of people like Alireza Beheshti is a sign of harsher incidents to come. He asked his supporters to remain calm.
An Iranian reformist Web site www.mowjcamp.com reported Wednesday that authorities arrested Beheshti, an opposition official who heads a panel investigating the abuse of protesters in post-election unrest.
Beheshti has said more than 70 people died during the unrest that followed the disputed re-election of President Mahmoud Ahmadinejad in June.
Also this week, authorities arrested Morteza Alviri, a former Tehran mayor and top aide to defeated presidential candidate Mehdi Karroubi. Karroubi has been a leading critic of Iran's post-election crackdown.
Iranian authorities closed Karroubi's offices on Tuesday.
The opposition leader's claims of detainee abuse have angered Iran's hardliners. The Iranian parliament speaker has rejected his claims as "baseless."
President Ahmadinejad was re-elected in a controversial June vote that saw Mr. Mousavi finish second. Government opponents say the election was fraudulent, and it set off days of street demonstrations.
Thousands of Iranians were arrested during the post-election protests.
Some information for this report was provided by AFP and Reuters.