In Cairo's Tahrir Square, a veiled woman ululated and young men danced while holding up a large plastic banner of the retiring, bearded engineer Mohammed Morsi, whom they were confident would be Egypt's new president.
The candidate's Muslim Brotherhood, which had representatives at polling stations across the country during the overnight count, has claimed that its candidate won 52% of the vote compared to 48% for his rival, former Prime Minister Ahmed Shafiq.
"I'm very, very happy," said Muhammad, a Morsi supporter. "The Brotherhood proved to be a successful political group. They will help us get jobs and we will see Islamic law."
The Shafiq campaign criticised the Islamist group for its victory announcement, saying it represented "bizarre behaviour" ahead of any official confirmation of the final result.
However, other media sources, such as the state-owned Al-Ahram news website, also put Mr Morsi ahead, though by an even narrower margin.
Some participants in the Tahrir Square rally felt their celebrations were overshadowed by the interim constitutional declaration made by the Supreme Council of the Armed Forces (Scaf) just as polls closed on Sunday.
It defined the prerogatives of both the next president and the ruling military after Egypt's supreme court dissolved the elected parliament - dominated by Islamists - last week.
"The aim is probably to deprive the new president of his proper powers," said Ahmed Abu Tahir, a middle-aged Morsi voter in Tahrir Square, reflecting the common suspicion.
"The military hands power to the military," was the headline in one of Egypt's main independent newspapers.
A lengthy news conference given by Scaf on Monday was intended to provide assurances. Maj-Gen Mamduh Shahin said provisional measures were needed because of recent developments.
"Circumstances were against us recently. The state of emergency is no longer in place and the constitutional court issued a ruling dissolving parliament, so we had to issue this supplementary constitutional declaration to arrange things for the coming period," he said.
The military confirmed that it had given itself legislative powers until a new parliament was functioning, but insisted these would be "restricted".
It also elaborated on its role overseeing the vital process of drawing up Egypt's new constitution. This saw repeated delays because of disputes between Islamists and liberals over the make-up of a panel to draft the document.
Scaf said that if the panel proved unable to complete its work, it would intervene. Steps leading to parliamentary elections would only start a month after a new constitution was ratified by a referendum.
Another senior member of the ruling council, Maj-Gen Mohammed al-Assar, told journalists that the new president would have the authority pick the new cabinet.
He maintained that power would be transferred to him in a "grand ceremony" at the end of the month.
The pledges did not quell anger from the Muslim Brotherhood and activists, who claimed the developments amounted to a complete military takeover.
"We reject this constitutional declaration. It is the continuation of the military coup that started after the revolution. Before, it was a soft coup, but not anymore," said Angy Hamdy of the 6 April Youth Movement.
"The military aims to have a symbolic president with a military constitution. We will continue our fight to end military rule."
Her group has planned protests in squares around the country late on Tuesday.
Another demonstration will also see MPs from the annulled parliament attempting to enter the building, which is currently surrounded by soldiers.
In its latest statement, the Brotherhood's political arm, which controlled the largest bloc of parliamentary seats, disputed the right of the military to take back power from the body.
"The parliament remains valid and holds legislative power and control," it said.
Analysts speculate that if official results confirm Mr Morsi has won the presidential election, a power struggle between the Brotherhood and the military - two of Egypt's strongest forces - looks likely, possibly bringing more of the turmoil that has characterised the process of democratic transition.
"At the moment, the military are heading for a confrontation with the Muslim Brotherhood, which is talking about cancelling the constitutional declaration," said Elijah Zarwan, a senior policy fellow in Cairo for the European Council on Foreign Relations.
"These latest moves have really raised the stakes."