Why did the Iranian leadership crack down on the election?

They'd already vetted both candidates anyway, and their system only allows the presidency limited power. Why didn't they let the election go down with more surface legitimacy, with peaceful demonstrations, re-counts, courts, lawyers, etc. There is nothing to lose. Did they panic? This seems like a big miscalculation. Instead of making it look more like an American election, they made it look like a sham.

Public Comments

1. When you have absolute power you worry about any threat to it. When folks question the 'election' process in the way they have/are then what might they do next? Authorities react. The opposition has already begun to pro-act. That will be what eventually comes through, unfortunately, 'eventually' is not a specific rate or period of time.
2. Iran's tenth presidential election was held on June 12, 2009. The President of Iran is the highest official elected by direct popular vote, but does not control foreign policy or the armed forces. Candidates have to be vetted by the Guardian Council, a twelve member body consisting of six clerics (selected by Iran's Supreme Leader), and six lawyers (proposed by the head of Iran's judicial system and voted in by the Parliament). The Islamic Republic News Agency, Iran's official news agency, announced that with two-thirds of the votes counted, incumbent Mahmoud Ahmadinejad had won the election with 66% of the votes cast, and that Mir-Hossein Mousavi had received 33% of the votes cast. The European Union and several western countries expressed concern over alleged irregularities during the vote, and some analysts and journalists from United States and Europe based media voiced doubts about the authenticity of the results. Meanwhile many OIC member states, as well as Russia, China, India, and Brazil, have congratulated Ahmadinejad on his victory. Mousavi issued a statement saying, "I'm warning that I won't surrender to this charade," and he urged his backers to fight the decision as well as to avoid committing acts of violence. Protests, in favour of Mousavi and against the alleged fraud, broke out in Tehran. Supreme Leader Ayatollah Ali Khamenei urged the nation to unite behind Ahmadinejad, labeling his victory as a "divine assessment". Mousavi lodged an official appeal against the result to the Guardian Council on June 14. On June 15, Khamenei announced there would be an investigation into vote-rigging claims, which would take seven to ten days. On June 16, the Guardian Council announced it will recount the votes. However, Mousavi stated that 14 million unused ballots were missing, giving a chance to manipulate the results.