Egypt's main opposition coalition has rejected President Mohammed Mursi's plan for a constitutional referendum this week, saying it risked dragging the country into "violent confrontation".
Mr Mursi's decision yesterday to retract a decree awarding himself wide powers failed to placate opponents.
His opponents accused him of plunging Egypt deeper into crisis by refusing to postpone the vote on a constitution shaped by Islamists.
The Front's main leaders - Nobel peace laureate Mohammed ElBaradei, former Foreign Minister Amr Moussa and leftist Hamdeen Sabahy - did not attend the event.
Hundreds of protesters milled around President Mursi's palace, despite tanks, barbed wire and other barriers installed last week.
Clashes between Islamists and their rivals killed seven people.
The Muslim Brotherhood, which propelled Mr Mursi from obscurity to power, urged the opposition to accept the referendum's verdict.
Islamists say the vote will seal a democratic transition that began when a popular uprising toppled Hosni Mubarak 22 months ago after three decades of military-backed one-man rule.
Their liberal, leftist and Christian adversaries say the document being fast-tracked through could threaten freedoms and fails to embrace the diversity of Egypt's 83m people.
Mr Mursi had given some ground yesterday when he annulled the fiercely contested decree issued on 22 November that gave him extra powers and shielded his decisions from judicial review.
But some measures taken under the decree remain in force and the president has insisted the referendum go ahead on 15 December.
Liberal opposition leader Ahmed Said earlier described the race to a referendum as an "act of war" against Egyptians.
Egypt is torn between Islamists, who were suppressed for decades, and their rivals, who fear religious conservatives want to squeeze out other voices and restrict social freedoms.
Many Egyptians just crave stability and economic recovery.
Brotherhood spokesman Mahmoud Ghozlan said the scrapping of Mr Mursi's decree had removed any reason for controversy.
The cancellation of Mr Mursi's decree, announced after a "national dialogue" yesterday boycotted by almost all the president's critics, has not bridged a deep political divide.
Prime Minister Hisham Kandil, a technocrat with Islamist leanings, said the referendum was the best test of opinion.
But opposition factions, uncertain of their ability to vote down the constitution against the Islamists' organisational muscle, want the document redrafted before any vote.