Since late 2011, more than 60 Arab activists have been arrested in Khuzestan province along the border with Iraq. At least two of the detainees have died, Human Rights Watch reported.
Local activists told the rights organization many people were rounded up for chanting anti-government slogans and for calling for a boycott of March parliamentary elections.
Joe Stork, deputy Middle East director at Human Rights Watch, said there was an information blackout in the province. Authorities, he said, should announce the reasons for the arrests and give detainees access to legal counsel.
"With the province under an information blackout and the history of secret convictions and executions, we have reason to be very worried about the people the authorities have been snatching up and carrying off there," he said in a statement from New York.
Human rights groups say Iranian authorities are cracking down on government critics in the months leading up to parliamentary elections.
Tehran recently hosted an international conference on the uprisings in the Middle East, which Tehran dubs the Islamic Awakening.