The crowd denounced the liberal opposition and its leaders, calling them undemocratic and accusing them of being loyalists of Hosni Mubarak, the authoritarian leader who was ousted in a popular uprising last year.
"Those protesting at the presidential palace are feloul (remnants of the Mubarak regime) and counter-revolutionaries," said Mohammed Abdel-Aziz, a young Islamist protester. "They don't want Islam."
Another pro-Morsi protester, school teacher Mohammed el-Hamoul, said Islamists "accepted democracy so we could reach power."
"Now those who claim to be democracy advocates lost faith in democracy when the Islamists rose to power," he said.
Several hundred Islamists also have set up camp across town outside a media complex that is home to several independent TV networks critical of Morsi and the Brotherhood. The Islamists have threatened to storm the facility.
The opposition, meanwhile, staged its rally in the nearby Heliopolis neighborhood, where tens of thousands of protesters gathered outside the presidential palace, pushing their demands that Morsi scrap the referendum.
"The people want the downfall of Brotherhood rule," the protesters chanted, alluding to the widespread notion that Mohammed Badie, the spiritual leader of the Muslim Brotherhood, is the country's actual ruler.
Troops belonging to the elite Republican Guards deployed outside the palace did not intervene.
Egypt's political crisis began on Nov. 22 when Morsi issued a decree granting himself — and the Islamist-dominated panel writing the constitution — immunity from judicial oversight or challenge. Those decrees sparked mass demonstrations, with opponents saying they were issued initially to protect the draft charter from the judiciary.
The constituent assembly then hurriedly approved the draft constitution in a marathon overnight session, prompting hundreds of thousands of the president's opponents to take to the streets in massive rallies — the largest from primarily secular groups since the uprising that toppled Hosni Mubarak last year.
Morsi's supporters responded with huge demonstrations of their own, which led to clashes in the streets that left at least six people dead and hundreds wounded.
From there, the violence rippled across the country, with at least two dozen attacks on offices of Morsi's Muslim Brotherhood, according to the group's leaders. Meanwhile, senior opposition figures, including former lawmakers, have been badly beaten by pro-Morsi Islamists.
Morsi has since rescinded the decree that gave him absolute powers, but did not meet the opposition's main demand and delay the referendum.
With four days to go before the vote, the opposition has yet to decide whether to campaign for a "no" vote or call for a boycott — something many see as a reflection of divisions within the opposition. The disparate opposition groups are led by reformist and Nobel Peace Laureate Mohamed ElBaradei, Egypt's former foreign minister and Arab League chief Amr Moussa, and leftist politician Hamdeen Sabahi.
Cracks in the opposition's unity first appeared last weekend when one of its leading figures, veteran opposition politician Ayman Nour, accepted an invitation by Morsi to attend a "national dialogue" meeting. On Monday, another key opposition figure, El-Sayed Badawi of the Wafd party, met Morsi at the presidential palace.
Badawi later issued a statement saying he remained loyal to the opposition's goals: scraping the draft charter and postponing the vote.
The opposition has rejected any dialogue with Morsi until he shelves the draft constitution and postpones the referendum. They had also demanded that Morsi rescind decrees giving him near absolute powers. He withdrew those powers on Saturday, but insisted that the referendum will go ahead as scheduled.
Anticipating unrest on the day of the referendum, Morsi has ordered the military to join the police in maintaining security and protecting state institutions until after the results of the vote are announced. The decree went into effect on Monday.
Associated Press writer Sarah El Deeb contributed to this report.
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