According to the letter, some male and female detainees in Kahrizak prison were said to have been subjected to serious sexual abuse.

Protests gripped Tehran and other Iranian cities after the June 12 presidential election, amid claims that the vote had been rigged in favour of incumbent President Mahmoud Ahmadinejad.

Iran's state media said about 30 people had been killed in relevant clashes, including some in custody.

Iran's judiciary said on Tuesday that 4,000 people were initially arrested by security forces during the post-pll protests and 3,700 were released later.