Hundreds of Brotherhood supporters had spent the night in Cairo's iconic Tahrir Square, having vowed to stay there until the election result is published. By early evening, their numbers had swelled to thousands.
"Morsi, Morsi, God is the Greatest," the protesters chanted in anticipation of a victory for their candidate, who says tallies provided by electoral officials show that he won.
Both Morsi and Shafiq have claimed victory in the election for a successor to Hosni Mubarak, sparking tensions between the rival camps that have deepened after the electoral commission delayed announcing the official outcome.
Across the city, in the Nasr City neighbourhood, thousands of Shafiq supporters held up pictures of their candidate and of military ruler Field Marshal Hussein Tantawi, chanting "the people and the army are one."
"Down with the rule of the Supreme Guide," protesters shouted, referring to the head of the Muslim Brotherhood.
A massive security plan has been put in place in the capital to prevent unrest when the result is announced on Sunday, an interior ministry official told AFP.
The delay in the announcement of the result of the June 16-17 run-off, initially scheduled for Thursday, has raised suspicions that the outcome of the election is being negotiated rather than counted.
As the Supreme Council of the Armed Forces (SCAF) and the Brotherhood clashed publicly over recent measures that consolidated the army's power, privately they have been talking behind the scenes, sources told AFP.
On Friday, the SCAF warned it would deal "with utmost firmness and strength" with any attempts to harm public interests, while the Brotherhood warned against tampering with the election results but said it had no intention of instigating violence.
The Brotherhood rejects a constitutional declaration by the military which strips away any gains made by the Islamist group since the popular uprising which forced Mubarak to stand down in February last year.
The document dissolves the Islamist-led parliament and gives the army a broad say in government policy and control over the new constitution. It was adopted just days after a justice ministry decree granted the army powers of arrest.
Even if Mursi wins the election, the changes leave the Brotherhood with no parliament, no say in the constitution and a powerless president.
"It's a problem which we are trying to resolve," one Brotherhood official said earlier.
The standoff comes amid conflicting reports over who would be declared the election winner and when the result would be announced.
"Morsi to be declared the winner today. Unless," read Saturday's headline of the independent daily Al-Shorouk.
"Shafiq is close to the presidential palace," said the liberal daily Al-Wafd on its front page.
The election 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 who fear the Brotherhood would stifle personal freedoms.
Shafiq ran on a strong law-and-order platform, pledging to restore security and stability.
Morsi, on the other hand, sought to allay the fears of secular groups and the sizeable Coptic Christian minority by promising a diverse and inclusive political system in Egypt.