AP Photo

TEHRAN — In silence they moved, a vast throng, hundreds of thousands of people, down the street called Revolution. President Mahmoud Ahmadinejad had called his opponents mere “dust.” Well, said one student, “We will blind him with our dust.”

Roger Cohen

This was the day followers of Mir Hussein Moussavi, the reformist candidate defeated in Iran’s disputed presidential elections, rose up en masse to protest the theft of their votes. “Quiet! Quiet!” they shouted, arms raised and fingers forming a “V” for victory that they pointed at a lonely police helicopter overhead.

Moussavi himself, not seen since the night of the election, appeared on Revolution Square, answering a question much debated here in recent days: Will he lead what...