UN Secretary-General Ban Ki-moon on Thursday called for the release of Iranian dissidents and journalists during his visit to Tehran.

Speaking at Tehran’s School of International Relations, Ban appealed to Iran’s leadership to release “opposition leaders, human rights defenders, journalists and social activists” from government prisons. Tehran had no immediate response.

“It is especially important for the voices of Iran’s people to be heard during next year’s presidential election,” Ban added. “Restricting freedom of expression and suppressing social activism will only set back development and plant the seeds of instability.”

Dozens of members of the Iranian opposition were arrested following the February and March 2011 protests, including former presidential candidate Mir Hossein Mousavi and dissident Mehdi Karroubi. Mousavi and Karroubi remain under house arrest.

German news agency DPA reported that there had been speculation that Ban would ask to visit Moussavi and Karroubi, but no such meeting had been arranged.

Human Rights Watch reported in May that dozens of students arrested in the wake of the 2009 Iranian Green Revolution remain imprisoned for “peacefully expressing political opinions.”