Tens of thousands of Mousavi supporters have gathered in central Tehran, pictures show.
So far the rally seems peaceful.  There have been reports of the police using tear gas against people marching.  Ahmedinejad apparently authorized the use of live ammunition against the protestors, but there are simply too many of them for the police to be willing to do something like that (thank God).  Mousavi has told the marchers that he is ready to take part in a new election; no word yet what Ahmedinejad has to say to that.