Thousands of people are gathering in Cairo for a protest against Egyptian President Mohammed Mursi, who last week granted himself sweeping new powers.
Ahead of the rally opposition activists clashed with police, who used tear gas.
Mr Mursi has sought to defuse the crisis by saying the scope of his new powers is limited, but his opponents want him to withdraw his decree.
On Monday, the Muslim Brotherhood cancelled a rally in support of the president to prevent violence.
Mona Amer, a spokesman for the opposition movement Popular Current, told Reuters: "We asked for the cancellation of the decree and that did not happen."
A group protesters camped on Cairo's Tahrir Square - the main focus of the revolution that ousted President Hosni Mubarak last year- told AFP news agency: "We will stay in Tahrir until Mursi cancels his declaration."
The decree, known as the constitutional declaration, said no authority could revoke presidential decisions.
There is a bar on judges dissolving the assembly drawing up a new constitution. The president is also authorised to take any measures to preserve the revolution, national unity or safeguard national security.
Critics said the decree was an attack on the judiciary. It has sparked violent protests in Cairo and across the country.
On Monday Mr Mursi told senior judges that the scope of the measure would be restricted to "sovereign matters", designed to protect institutions.
The president's Muslim Brotherhood said it was postponing its own demonstration to avoid "public tension".
The BBC's Jon Leyne in Cairo says the postponement is another sign that the government wants to defuse confrontation, but it remains to be seen whether it ends the days of angry and sometimes violent protests.
Several prominent opposition leaders, including Nobel Peace Prize laureate Mohamed ElBaradei, have said they will not engage in dialogue until the president rescinds the measure, known as the constitutional declaration.
