His supporters and other Islamists chanted, "The people support the president's decree!" and pumped their fists in the air.
"God will humiliate those who are attacking our president, Mohammed Morsi," said ultraconservative cleric Mohammed Abdel-Maksoud.
"Whoever insults the sultan, God humiliates him," he added.
The state media described Morsi's decrees as a "corrective revolution," and supporters cast them as the only way to break through the political deadlock over drafting the constitution.
Mustafa Kamel el-Sayyed, a Cairo University political science professor, said Morsi may be confident that the U.S. won't pressure him on his domestic moves.
"The U.S. administration is happy to work with an Islamist government (that acts) in accordance with U.S. interests in the region," including preserving the Egyptian-Israel peace deal, he said.
With his decrees, Morsi was playing to widespread discontent with the judiciary. Many — even Brotherhood opponents — contend Mubarak-era judges and officials failed to prosecute the old regime's top officials and security forces strongly enough for crimes, including the killing of protesters.
Morsi fired the controversial prosecutor general and created "revolutionary" judicial bodies to put Mubarak and some of his top aides on trial a second time for the killings. Mubarak was sentenced to life in prison for failing to stop police from shooting at protesters, but many were angry he was not found guilty of actually ordering the crackdown during the uprising.
Some among Egypt's liberal, leftist and secular forces saw the edicts as an opportunity to galvanize an opposition that has been chronically fragmented.
Sameh Makram Obeid, a leader in the liberal Dustour Party, said Morsi's declarations are a "blessing" because they energized his opponents.
"The solution is civil disobedience," he said, echoing other activist leaders. "The separation of powers is gone completely."
"We are in a state of revolution. He is crazy if he thinks he can go back to one-man rule," said one protester in Tahrir, Sara Khalili.