TEHRAN  The Iranian police commander, in green uniform, walked up Komak Hospital Alley with arms raised and his small unit at his side. “I swear to God,” he shouted at the protesters facing him, “I have children, I have a wife, I don’t want to beat people. Please go home.”

A man at my side threw a rock at him. The commander, unflinching, continued to plead. There were chants of “Join us! Join us!” The unit retreated toward Revolution Street, where vast crowds eddied back and forth confronted by baton-wielding Basij militia and black-clad riot police officers on motorbikes.

Dark smoke billowed over this vast city in the late afternoon. Motorbikes were set on fire, sending bursts of bright flame skyward. Ayatollah Ali Khamenei, the supreme leader, had used his Friday sermon to declare high noon in Tehran, warning of “bloodshed and chaos” if protests over a disputed election persisted.

He got both on Saturday  and saw the hitherto sacrosanct authority of his office challenged as never before since the 1979 revolution birthed the Islamic Republic and conceived for it a leadership post standing at the very flank of the Prophet. A multitude of Iranians took their fight through a holy breach on Saturday from which there appears to be scant turning back.