Gary Sick, the dean of U.S. Iran experts, cuts to the heart of the matter:
The Iranian opposition, which includes some very powerful individuals and institutions, has an agonizing decision to make. If they are intimidated and silenced by the show of force (as they have been in the past), they will lose all credibility in the future with even their most devoted followers. But if they choose to confront their ruthless colleagues forcefully, not only is it likely to be messy but it could risk running out of control and potentially bring down the entire existing power structure, of which they are participants and beneficiaries.
He goes on:
In their own paranoia and hunger for power, the leaders of Iran have insulted their own fellow revolutionaries who have come to have second thoughts about absolute rule and the costs of repression, and they may have alienated an entire generation of future Iranian leaders. At the same time, they have provided an invaluable gift to their worst enemies abroad.
However this turns out, it is a historic turning point in the 30-year history of Iran’s Islamic revolution. Iranians have never forgotten the external political intervention that thwarted their democratic aspirations in 1953. How will they remember this day?
By the way, you can also follow someone claiming to be Mousavi's Twitter feed here. His latest post reads "Dear Iranian People, Mousavi has not left you alone, he has been put under house arrest by Ministry of Intelligence." No word from any media on whether this is true or not.
There's also a link to an English translation of Mousavi's letter to the Iranian people. It begins:
The reported results of the 10th Iranians residential [sic] Election are appalling. The people who witnessed the mixture of votes in long lineups know who they have voted for and observe the wizardry of I.R.I.B (State run TV and Radio) and election officials. Now more than ever before they want to know how and by which officials this game plan has been designed. I object fully to the current procedures and obvious and abundant deviations from law on the day of election and alert people to not surrender to this dangerous plot. Dishonesty and corruption of officials as we have seen will only result in weakening the pillars of the Islamic Republic of Iran and empowers lies and dictatorships.
I am obliged, due to my religious and national duties, to expose this dangerous plot and to explain its devastating effects on the future of Iran. I am concerned that the continuation of the current situation will transform all key members of this regime into fabulists in confrontation with the nation and seriously jeopardize them in this world and the next.
I advise all officials to halt this agenda at once before it is too late, return to the rule of law and protect the nation’s vote and know that deviation from law renders them illegitimate. They are aware better than anyone else that this country has been through a grand Islamic revolution and the least message of this revolution is that our nation is alert and will oppose anyone who aims to seize the power against the law.
See also Mousavi1388's Flickr account, which includes dozens of photos of today's riots.
7:58 PM ET: NIAC reports: "Through Facebook we have received news that Mir Hossein Mousavi, Mehdi Karroubi, and Gholamhossein Karbaschi are under house arrest." The usual caveats for unconfirmed reports apply.