wikipedia-Marcello Casal Jr\ABr
Iranians vote for a president to succeed Mahmoud Ahmadinejad after two terms in office.
In the 2009 election, Ahmadinejad was declared a winner despite his challengers' allegations of fraud. Widespread protests were crushed by the authorities and the leaders of the opposition have been under house arrest ever since.
During his latest visit to Tehran, UN chief Ban Ki-moon called on Iran to release its political prisoners and warned that restricting the freedom of expression ahead of the elections would only lead to political instability.
"I have urged the authorities during my visit this time to release opposition leaders, human rights defenders, journalists and social activists to create the conditions for free expression and open debate," he said.