Backers of Egypt's President Mohammed Morsi are to hold a mass rally to support his sweeping new powers and the drafting of a constitution opponents say is too Islamist.
The demonstration in Cairo comes after days of rival protests by supporters and opponents of the president.
His opponents are angry that the draft constitution was hastily backed by the Islamist-dominated assembly on Friday.
Egypt's top court is to rule whether the assembly should be dissolved.
Senior judges have been in a stand-off with the president since he granted himself sweeping new powers last week.
'Fall of the regime'
Saturday's mass rally in support of Mr Morsi near Cairo University has been called by the Muslim Brotherhood and other Islamist parties.
They say a huge turnout would show that the president's recent moves are supported by the public.
Under an emergency decree issued last week, Mr Morsi's decisions cannot be revoked by any authority, including the judiciary, until the new constitution has been ratified and a fresh parliamentary election held.
It also states that the courts cannot dissolve the constituent assembly.
Mr Morsi says he will give up his extraordinary powers once the new constitution is approved by a referendum.
The president is expected to ratify the draft later on Saturday before putting it to a popular vote.
The BBC's Jon Leyne in Cairo says the president may announce the referendum on Saturday and hold it in as little as two weeks' time.
Our correspondent says the key question will be whether the opposition can mobilise its support and get it to the ballot boxes to vote in the referendum.
On Friday, tens of thousands of people opposed to the president rallied in Cairo's Tahrir Square.
Demonstrators chanted slogans, including "The people want the fall of the regime!" - one of the rallying cries against former President Hosni Mubarak, who was toppled last year.
In the city of Alexandria, supporters and opponents of the president clashed on the streets.
The extent of Mr Morsi's new powers has raised fears that he might become a new strongman.
UN Human Rights Commissioner Navi Pillay has written to the president, asking him to reconsider his decree.
In her letter, Ms Pillay "warned that approving a constitution in these circumstances could be a deeply divisive move", her spokesman said.
Mr Morsi's decree of 22 November gave the 100-member constituent assembly until January to complete the draft constitution.
Opponents filed 43 separate lawsuits challenging the process.
When the Supreme Constitutional Court, Egypt's highest judicial authority, said it would soon rule on the lawsuits, supporters of the president in the assembly decided to pass a rushed draft to head off the threat of dissolution.
During a marathon session that began on Thursday and continued through the night, the assembly voted on and passed all 234 articles.
Among the historic changes to Egypt's system of government, the draft limits the amount of time a president can serve to two four-year terms.
It also introduces some civilian oversight of the military establishment.
The draft keeps in place an article defining "principles of Sharia", or Islamic law, as the main source of legislation.