The letter comes on the eve of the 31st anniversary of Iran's Islamic revolution.

Opposition activists have called for anti-government protests on Thursday but the authorities have pledged to crack down hard if they take place.

Earlier the government announced the arrest of several people it said were preparing to disrupt official rallies.

Widespread unrest erupted in the wake of the disputed presidential vote in June last year, amid allegations that the election, resulting in Mahmoud Ahmadinejad winning a second term, was rigged.