Anti-Morsi dissidents plan a sit-in in Tahrir Square, site of the large protests that brought down President Hosni Mubarak in 2011. They began putting up tents Friday, preparing for an indefinite stay, Ahram Online reported.
Abdel-Rahman El-Barr said Egypt is "one nation" and warned dissidents Morsi heads the legitimate government, Ahram Online said.
He spoke at a pro-Morsi rally in Nasr City.
"We will not accept that the democratic choice of the people is overturned," El-Barr said. "All state institutions, including the army and police, are working to protect the legitimate government."
Supporters were bused to the Nasr City mosque from other cities and Cairo districts.
The Legitimacy Support Alliance, a new umbrella group composed of 11 Islamic parties, is coordinating the pro-Morsi protests and sit-ins.
The Cairo Traffic Authority positioned metal barriers in the roads to the mosque to help prevent clashes between supporters and other citizens.
At least three people have been killed and more than 400 injured in violence involving pro- and anti-Morsi groups in two days this week.
The anniversary of Morsi's inauguration is Sunday.
During a televised speech Wednesday, Morsi repeatedly threatened to purge holdovers from the Mubarak regime, and offered no major concessions to those calling for his ouster.
"This is the message he is sending, a threat pretty much that his tolerance and patience for so-called subversive acts and extra-constitutional activities will no longer stand," Yasser el-Shimy, an analyst with the International Crisis Group, told The New York Times. "If you would rather work outside the system, we are going to come after you."
Opposition groups maintain Morsi has failed to bring any of the ideals from the country's revolution to fruition.
Organizers of an anti-Morsi "Rebel" petition drive say they have collected 15 million signatures endorsing its call for him to leave office.