But the White House was notably silent after it had earlier this week extolled the emerging relationship between President Obama and Mr. Morsi and credited a series of telephone calls between the two men with helping to mediate the cease-fire in Gaza.

For Mr. Morsi, who seemed to be saying to the nation that it needed to surrender the last checks on his power in order to save democracy from Mubarak-era judges, the challenge was to convince Egyptians that the ends justified his means.

But even as he tried, thousands of protesters marched to condemn his decision. Clashes broke out between the president’s supporters and his critics, and near Tahrir Square, the riot police fired tear gas and bird shot as protesters hurled stones and set fires.

Since Thursday, when Mr. Morsi issued the decree, the president and his supporters have argued that he acted precisely to gain the power to address the complaints of his critics, including the families of protesters killed during the uprising and its aftermath.

By placing his decisions above judicial review, the decree enabled him to replace a public prosecutor who had failed to win convictions against senior officers implicated in the killings of protesters.

The president and his supporters also argued that the decree insulated the Constituent Assembly, which is drafting the constitution, from meddling by Mubarak-era judges.