Likely emboldened by the Guardian Council's decision to probe charges of election improprieties, Mir Hossein Mousavi and an estimated 100,000 supporters defied a ban on protest demonstrations to show their strength in Tehran today. The event was marred when members of what is called a "pro-government militia" fired into the crowd, reportedly killing at least one person. President Ahmadinejad has cancelled a scheduled trip to Russia in order to remain in the country, which is probably a smart move by the presumptive winner.
From the MSNBC account, the gunmen may have been members of the basij, which is a student paramilitary group with reactionary leanings. The "pro-government" label should be used with care because of the distinction between Ahmadinejad as head of government and Ayatollah Khamanei as head of state. The basij, from what I've read about them, are mainly pro-Ahmadinejad, though I doubt whether they see any conflict between that and loyalty to the Supreme Leader or the Guardian Council. The president has reportedly encouraged the growth of the basij during his first term, and they seem like the people most inclined to get rough in his defense. They could have acted on their own initiative, or with Ahmadinejad's connivance, or with approval from the very top level. This is the sort of scenario, however, where conspiracy theorists are most likely to cry "agent provocateur!" and suggest that the basij, Ahmadinejad, or the government as a whole are being framed.
Whether the Guardian Council's inquiry into the election can be taken seriously depends on how tight Ahmadinejad is with Khamenei. The election was fought mostly over domestic economic issues, and I don't know if the ayatollah has a preference here. The other main issue was Ahmadinejad himself as the image of Iran to the rest of the world. Mousavi portrayed him as an embarrassment, but supporters endorsed the president's confrontational stance as a way to stick it to The Man, i.e. the Great Satan and his pals. It's not clear whether Mousavi would offer more than cosmetic changes when it comes to foreign relations. The question is whether the Guardians would have any reason to screw Mousavi, but that question itself assumes that Ahmadinejad has already screwed his opponent. American observers may be inclined to believe that Khamenei is automatically complicit in any Election Day screw-job, but just as we shouldn't assume that Mousavi has been screwed, we shouldn't assume that the Supreme Leader was in on a conspiracy against the opposition, or that he would do nothing about it. The proof of a free election is not that the good guys win, or that all the candidates are good guys. Iran may end up an enemy of the U.S. despite all democracy, but that possibility shouldn't lead us to assume that the Islamic Republic is more of a tyranny than it probably is -- which is bad enough for many people.