Tuesday's turnout was an unprecedented show of strength by the mainly liberal and secular opposition, which has been divided and uncertain amid the rise to power of the Brotherhood over the past year. The crowds were of all stripes, including many first-time protesters.
"Suddenly Morsi is issuing laws and becoming the absolute ruler, holding all powers in his hands," said Mona Sadek, a 31-year-old engineering graduate who wears the Islamic veil, a hallmark of piety. "Our revolt against the decrees became a protest against the Brotherhood as well."
"The Brotherhood hijacked the revolution," agreed Raafat Magdi, an engineer who was among a crowd of some 10,000 marching from the Cairo district of Shubra to Tahrir to the beat of drums and chants against the Brotherhood. Reform leader Mohammed ElBaradei led the march.
"People woke up to (Morsi's) mistakes, and in any new elections they will get no votes," Magdi said.
Many in the crowd said they were determined to push ahead with the protests until Morsi retreats. A major concern was that Islamists would use the decree's protection of the constitutional assembly to drive through their vision for the next charter, with a heavy emphasis on implementing Shariah, or Islamic law. The assembly has been plagued with controversy, and more than two dozen of its 100 members have quit in recent days to protest Islamist control.
"Next Friday will be decisive," protester Islam Bayoumi said of the upcoming rally. "If people maintain the same pressure and come in large numbers, they could manage to press the president and rescue the constitution."
A fellow protester, Saad Salem Nada, said of Morsi: "I am a Muslim and he made me hate Muslims because of the dictatorship in the name of religion. In the past, we had one Mubarak. Now we have hundreds."
Even as the crowds swelled in Tahrir, clashes erupted nearby between several hundred protesters throwing stones and police firing tear gas on a street leading to the U.S. Embassy. Clouds of tear gas hung over the area, where clashes have broken out for several days, fueled by anger over police abuses.
A photographer working for the AP, Ahmed Gomaa, was beaten by stick-wielding police while covering the clashes. Police took his equipment and Gomaa was taken to a hospital for treatment.
Rival rallies by Morsi opponents and supporters turned into brief clashes in the Mediterranean city of Alexandria, where anti-Morsi protesters broke into the local office of the Muslim Brotherhood, throwing furniture out the windows and trying unsuccessfully to set fire to it. Protesters also set fire to Brotherhood offices in the city of Mansoura.
Morsi's supporters canceled a massive rally planned for Tuesday in Cairo, citing the need to "defuse tension." Morsi's supporters say more than a dozen of their offices have been ransacked or set ablaze since Friday. Some 5,000 demonstrated in the southern city of Assiut in support of Morsi's decrees, according to witnesses there.
So far, there has been little sign of a compromise. On Monday, Morsi met with the nation's top judges and tried to win their acceptance of his decrees. But the move was dismissed by many in the opposition and the judiciary as providing no real concessions.
Saad Emara, a senior Muslim Brotherhood member, said Morsi will not make any concessions, especially after the surge of violence and assaults on Brotherhood offices.
Emara accused the opposition "of resorting to violence with a political cover," claiming that former ruling party and Mubarak-era businessmen were hiring thugs to attack Brotherhood offices with the opposition's blessing.
"The story now is that the civilian forces are playing with fire. This is a dangerous scene."
Associated Press writer Hamza Hendawi in Cairo contributed to this report.