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Two weeks after the contested results of Iran's Presidential elections led to widespread street riots and demonstrations across the country, the Islamic Republic pronounced its harshest threat yet to protesters. At the official ceremony for Friday prayers, Ayatollah Ahmad Khatami, a hard-line cleric who often delivers the sermon, said those who agitate on the streets were "waging war against God," a crime that carries the death sentence.

It was the latest example in which government forces have tightened their control over and heightened their rhetoric against opposition supporters of the defeated presidential candidate Mir-Hossein Mousavi. Demonstrations and rallies have ground to a halt as the heavy presence of police, Revolutionary Guards officers, plainclothes intelligence...