Egypt's opposition rejected an offer of compromise by President Mohammed Mursi to end the country's crisis over a new constitution on Sunday, and called for mass street protests on Monday.
A fresh rally was under way around the presidential palace on Sunday night as leaders of the liberal and secular opposition to Mursi's Muslim Brotherhood set up a final showdown before Saturday's referendum on the constitution.
The Islamists said they would hold a rival demonstration on Tuesday, throwing up the potential for clashes. Mursi agreed on Saturday night to drop a constitutional decree giving himself powers beyond judicial scrutiny, but replaced it with a new one that insisted that the referendum would go ahead despite calls for a delay.
A moderate Islamist politician involved in talks with the Brotherhood said there would be an opportunity later to revise controversial articles of the draft constitution, which the opposition says does not guarantee basic rights.
Opposition groups refused to back down and held a press conference to denounce the referendum and Mursi's repeated decrees. "We do not recognise the draft constitution because it does not represent the Egyptian people," said a statement by the opposition coalition National Salvation Front, adding that, "We reject the referendum which will certainly lead to more division and sedition."
The April 6 Movement, a Left-wing group that was behind the revolution against Hosni Mubarak, called the compromise "a continuation of deception in the name of law and legitimacy". The Brotherhood is confident it has enough support to win the referendum, which will be decided on a straight majority. It believes that the liberals represent a minority view, and that most Egyptians are happy to see the emphasis on Islam that runs through parts of the draft.
"The seculars and liberals are mobilising people to bring down the Islamic state," said Mohammed Bishawi, a long-standing Muslim Brotherhood member, at a counter-rally at the organisation's headquarters on the outskirts of Cairo.
The building was ransacked by an angry mob on Thursday night, as were other Brotherhood-linked offices around the country, and the organisation was staging a symbolic demonstration to protect it.
Bishawi claimed that the wave of opposition to Mursi in the past two weeks, since he announced his new powers, was "manufactured by the media". "We, the Islamic powers, chosen by democratic election, want to prove to the world that the Egyptian people are with president Mursi and their new constitution," he said. "The liberals don't have any real existence on the street."
Both sides are involved in a high-stakes gamble. The opposition risks making itself look irrelevant if the constitution is passed by a large majority. But if the Brotherhood loses the referendum or has a cripplingly low turnout, it will be a damaging blow to its assumption that political Islam is the dominant voice of Egyptian society.
Defeat would also suggest the public accepted the opposition view that the Brotherhood is showing increasingly authoritarian tendencies. The prosecution service yesterday referred to investigators a Brotherhood claim that opposition leaders, judges and journalists were conspiring to overthrow the government.
If there is a "no" vote in the referendum, a new 100-member constitutional assembly will have to be elected within three months, a new constitution agreed within six months and put to a referendum within a month after that. Only after that can there be new parliamentary elections, by which time the Brotherhood may have lost the popularity it had when it won elections convincingly earlier in the year.