Thousands packed Cairo's iconic Tahrir Square on Friday to denounce a power grab by the ruling military, as the nation nervously awaited the results of the first post-Mubarak presidential election.
Members and supporters of the powerful Muslim Brotherhood gathered in the square for the protest, which was to be joined later in the afternoon by several secular movements.
Protesters used umbrellas, newspapers and hats to shield themselves from the brutal sun, as they chanted "down with military rule."
Preacher Mazhar Shahin, dubbed the preacher of the revolution, called on the military to cancel the constitutional declaration that gave it sweeping powers and to reverse a decision that grants the army powers of arrest.
He warned that any hint of fraud in the awaited election results would be "confronted by the people by all means that are peaceful."
State television said the ruling Supreme Council of the Armed Forces (SCAF) was to issue a statement at 1:30 pm (1130 GMT), amid mounting anger over the perceived threat to the fragile democratic gains made since the uprising that ousted Hosni Mubarak. It did not elaborate.
The Muslim Brotherhood insists its candidate Mohamed Morsi has won the deeply divisive election, providing copies of official polling station tallies as proof.
But the claim has been strongly rejected by his rival Ahmed Shafiq, a former premier under Mubarak.
In a speech on Thursday, Shafiq said he was confident he would be declared the "legitimate" president, raising fears of unrest in a country exhausted by 18 months of political upheaval and insecurity.
The race has polarised the nation, dividing those who fear a return to the old regime under Shafiq’s leadership from others who want to keep religion out of politics and fear the Brotherhood would stifle personal freedoms.
A delay in the run-off results, which had been due on Thursday, widely raised suspicions that the result was being negotiated rather than counted.
The Muslim Brotherhood has been has been holding crisis talks with the country's political forces for days, as it braced for a showdown with the military.
In the months following Mubarak's resignation in February last year, the Brotherhood lost the support of revolutionary groups, who accused the Islamist group of trying to monopolise politics.
But the power grab by the army has given the Brotherhood a new opportunity to reach out to them.
Morsi also spoke by telephone with Nobel laureate and reform campaigner Mohamed ElBaradei, as well as with former presidential hopeful Abdel Moneim Abul Fotouh ahead of the protests, said the Freedom and Justice Party (FJP), the Brotherhood's political arm.
The Brotherhood would announce "a national project to defend the revolution," it said.
The April 6 movement, which helped launch last year's uprising that toppled Mubarak, said it would join Friday's protests to express "its rejection of the constitutional declaration" and "continue to fight for the goals of the revolution."
The National Front for Justice and Democracy said it rejected the constitutional declaration "which constitutes a military coup."
A recent set of measures consolidating the army's power has infuriated pro-democracy groups and raised concerns abroad.
The ruling SCAF assumed legislative powers after a court ordered the Islamist-led parliament dissolved and issued decrees giving the army powers of arrest and a broad say in government policy, rendering the president's post toothless.
Protesters have been in Tahrir Square since the constitutional document was issued on Sunday.