The Guardian has a reporter on the ground:
The numbers at today's rally are hard to gauge, but our correspondent Saeed Kamali Dehghan, reckons there could be as many as one million people there. He said the demonstration is bigger than Monday's rally. Many are wearing black and carrying photos of those who died. Some are carry placards calling for a new election not a recount. Saeed pointed out that the rally has taken in place in South Tehran where Ahmadinejad claimed to have had a lot of support.
(Photo: our mysterious street artist on the streets of Tehran strikes again. You can see more of his/her work here. The text reads: "Where Is My Vote?")