A fascinating account of the events by Muhammad Sahimi, interviewing Mousavi's spokesman, Mohsen Makhbalbaf:
According to Mr. Makhbalbaf, in the early hours after voting had ended, the Interior Ministry had called Mr. Mousavi’s campaign headquarters to inform them that Mr. Mousavi would be the winner and, therefore, Mr. Mousavi must prepare a victory statement. Mr. Mousavi was, however, asked by the Ministry not to boast too much, in order not to upset Mr. Ahmadinejad’s supporters. Many of the president’s supporters are among the ranks of the Basij militia, and thus armed. According to Mr. Makhbalbaf, the Supreme Leader, Ayatollah Ali Khamenei, was also informed of the developments. He also recommended a “good management” of the victory statement, meaning not boasting greatly about the victory, because that would be in Iran’s national interests and stability.
At the same time, the reformist newspapers were also informed that they can prepare their Saturday edition to declare Mr. Mousavi the winner, but were not allowed to use the word pirouzi (victory) in their articles, in order not to upset Mr. Ahmadinejad’s supporters. One reformist newspaper prepared its front page with the title, “People took back the flag of their country [from Mr. Ahmadinejad].” But, just a few hours later, a center that had been set up by Mr. Mousavi in Gheytarieh (in northern Tehran) for monitoring the election and vote counting, was attacked by armed security agents. They ransacked the center, destroyed computers, and attacked the staff. Supporters of Mr. Mousavi intervened and arrested 8 security agents. The police was called to take them to prison, but the police released the attackers. According to Mr. Makhbalbaf, the central headquaters of Mr. Mousavi’s campaign was also surrounded by security forces, as was the Interior Ministry building. Then, new data began to be released by the Ministry, indicating that Mr. Ahmadinejad had won the elections decisively. Mr. Makhbalbaf then declared that, “I have been authorized by Mr. Mousavi’s campaign to officially declare that a political coup has taken place, in order to declare Mr. Ahmadinejad the victor.”