Here is some video of Sunday's rally near the Ghoba mosque in Tehran.
Thousands were reported to have attended, and judging by what I can see here, there's no reason to doubt that figure. I can hear the crowd chanting "Karroubi" and "Moussavi" but I don't understand the rest of it.
I've read reports of another mass protest near Vali Asr Square, but I cannot yet verify this.