Already, the country's powerful judges have said they will not take on their customary role of overseeing the vote, thus robbing it of much of its legitimacy.
Morsi was in the presidential palace conducting business as usual as the protesters gathered outside. He left for home through a back door as the crowds continued to swell, according to a presidential official who spoke on condition of anonymity because he was not authorized to speak to the media.
The official said Morsi left on the advice of security officials to head off "possible dangers" and to calm the protesters. Morsi's spokesman, however, said the president left the palace at the end of his normal work day, through the door he routinely uses.
The protest was peaceful except for a brief outburst when police used tear gas to prevent demonstrators from removing a barricade topped with barbed wire and converging on the palace.
Soon after, with the president gone, the police abandoned their lines and the protesters surged ahead to reach the palace walls. But there were no attempts to storm the palace, guarded inside by the army's Republican Guard.
Protesters also commandeered two police vans, climbing atop the armored vehicles to jubilantly wave Egypt's red, white and black flag and chant against Morsi. The protesters later mingled freely with the black-clad riot police, as more and more people flocked to the area to join the demonstration.
The protesters covered most of the palace walls with anti-Morsi graffiti and waved giant banners carrying images of revolutionaries killed in earlier protests. "Down with the regime" and "No to Morsi," they wrote on the walls.
"He isn't the president of all Egyptians, only of the Muslim Brotherhood," said protester Mariam Metwally, a postgraduate student of international law. "We don't feel like he is our president."
A giant poster emblazoned with an image of Morsi wearing a Pharaonic crown was hoisted between two street light posts outside the presidential palace. "Down with the president. No to the constitution," it declared.