Pro-government demonstrators took to Cairo's streets in support of a new draft constitution that was approved a day earlier after a long 20-hour vote. The marches started Saturday following afternoon prayer, according to Al Jazeera.
The news network reported "Islamist and nationalist revolutionary forces" left the mosques according to the Freedom and Justice Party, an arm of the Muslim Brotherhood.
The Wall Street Journal said President Mohammed Morsi announced a referendum on that new constitution would be held in the coming weeks on Dec. 15.
"I tell our great people, the opponent before the supporter, help me with all your strength to fulfill the responsibility that you have entrusted upon me to manage the nation's affairs," Morsi said in a televised speech.
CNN said the opposition remains unsatisfied.
"The struggle will continue... (Morsi) put to referendum a draft constitution that undermines basic freedoms and violates universal values," Mohamed ElBaradei, head of Egypt's Constitution Party, tweeted.
The Journal estimated "hundreds of thousands of Islamist supporters" took to the streets to support Morsi on Saturday.
The rallies come a day after tens of thousands flooded Tahrir Square against the newly-elected president.
Demonstrators held up banners saying "Together [with Morsi] to save the revolution," Al Jazeera reported. Saturday's demonstrations also follow violent protests Friday in Alexandria.