Joe Klein agrees that the arrests of Mousavi and Karroubi show the regime to be "slightly shaky":

The international sanctions are hurting the Iranian economy; the probable US/Israeli sabotage of the Bushehr nuclear reactor may be just the tip of the problems now MOUSAVI KARROUBI POSTERplaguing the Iranian program; and, of course, the democracy movements sweeping the region are a reminder of the freedoms Iranian people don't have.

The Obama Administration is being a bit more aggressive in its condemnations of the Iranian government these days, which is probably a reflection of its desire to show that it's on the side of the people in the streets throughout the middle east. But I'm still wary about the Administration getting too vocal in support of the Green movement: it may well give the Supreme Leader and his thugs a green light to really rip into the demonstrators who may take to the streets [today]--and, believe me, the Iranian government has the power and the religious army to wreak havoc. The Revolutionary Guard and basij (the religious police) do not at all resemble the Egyptian Army that refused to attack the protesters or the Libyan Army that splintered. They consider the protesters heretics, enemies of God, and have few second thoughts when it comes to brutalizing them.

Tehran Bureau and IranNewsNow are live-blogging the day's developments. As is Scott Lucas, who begins:

Even before the arrests, while Mousavi, the Karroubis, and Rahnavard were confined to their house, the opposition had called on its supporters to march today, following up the rallies of 14 and 20 February. Now that call takes on an added significance, as a sign of defiance of the regime's escalated repression. 

The opposition has faced the challenge of pulling this off despite being scattered by detentions and limited in communications. Still, there is chatter this morning that people know of the call and will respond. A source tells us that of a family in Iran --- not in Tehran, but in the countryside --- who, for the first time, made a political statement. They did so in code, speaking about a rainstorm and adding, "Watch the forecast as it is going to be huge."

And there is a third important development which is accelerating. President Ahmadinejad's supporters are pushing their campaign to remove former President Hashemi Rafsanjani as head of the Assembly of Experts tomorrow. Significantly, the top story in the pro-Ahmadinejad Islamic Republic News Agency this morning is an interview with a spokesman of the conservative Society of Combatant Clergy who says, "Ayatollah Mahdavi Kani is the best choice for President of the Assembly."

Mahdavi Kani confirmed yesterday that he would stand for election tomorrow.

(Image via Josh Shahryar)

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