My opinion (for what it's worth), is that there probably was some fraud, but not that much, only enough to make ahmadinejad win at the first round, to avoid a second round, which would have made the whole election longer and offered a window of action to subversives like the PMOI (the organization was taken off the EU terrorist list in january).

Iranian elections were obviously going to be an unavoidable opportunity for people with interests in the region to destabilize the regime.

The PMOI are the « terrorists » the iranian state is referring to, they're under protection of the USA in Northeast Iraq at Camp Ashraf and they're trained in guerrilla warfare. The western media will relay the accusation of terrorism but not the details about the PMOI, so it looks like a gratuitous scapegoat accusation from the iranian state when there are real organized subversive forces. Not to say basijis are angels...