Not my area of expertise, of course, so I'll out-source to others who know what they're talking about. Like Juan Cole, who sees evidence of fraud:
I am aware of the difficulties of catching history on the run. Some explanation may emerge for Ahmadinejad's upset that does not involve fraud. For instance, it is possible that he has gotten the credit for spreading around a lot of oil money in the form of favors to his constituencies, but somehow managed to escape the blame for the resultant high inflation.
But just as a first reaction, this post-election situation looks to me like a crime scene. And here is how I would reconstruct the crime.
As the real numbers started coming into the Interior Ministry late on Friday, it became clear that Mousavi was winning. Mousavi's spokesman abroad, filmmaker Mohsen Makhbalbaf, alleges that the ministry even contacted Mousavi's camp and said it would begin preparing the population for this victory.
The ministry must have informed Supreme Leader Ali Khamenei, who has had a feud with Mousavi for over 30 years, who found this outcome unsupportable. And, apparently, he and other top leaders had been so confident of an Ahmadinejad win that they had made no contingency plans for what to do if he looked as though he would lose.
They therefore sent blanket instructions to the Electoral Commission to falsify the vote counts.
This clumsy cover-up then produced the incredible result of an Ahmadinejad landlside in Tabriz and Isfahan and Tehran.
Cole has a list of suspicious election results at the link above. But Nate Silver is not convinced of fraud allegations, and argues the graph below (which has been making the rounds) does not necessarily indicate foul-play:
The NY Times' Lede blog has been updating all day as new developments emerge, as has niacINsight, which has recently received Facebook messages that Mousavi has been placed under house arrest. Sullivan is posting any information he gets his hands on, including this vivid imagery:
"My next door neighbor is an Iranian immigrant who came here in 1977. He just received a SAT phone call from his brother in Tehran who reports that the rooftops of nighttime Tehran are filled with people shouting 'Allah O Akbar' in protest of the government and election results. The last time he remembers this happening is in 1979 during the Revolution. Says the sound of tens of thousands on the rooftops is deafening right now."
I have no idea how reliable any of this is, so take it all with a grain of salt. In fact, Ahmadinejad was the favorite before voting began, and probably would have won the election if everything was on the up-and-up. But that doesn't mean that the vote wasn't tampered with. There are enough irregularities to cast a suspicious eye toward Tehran.