Egypt's Islamist President-elect Mohamed Morsi took an informal oath of office on Friday before tens of thousands of supporters in Cairo's Tahrir Square, in a slap at the generals trying to limit his power.

"I swear by God that I will sincerely protect the republican system and that I respect the constitution and the rule of law," Morsi said to wild cheers from the crowd, many of whom were followers of his once-banned Muslim Brotherhood.

"I will look after the interests of the people and protect the independence of the nation and the safety of its territory," said the bearded Morsi, in an open-necked shirt and suit.

Morsi swore a symbolic oath before the crowd, He promised to be a "president for all Egyptians", adding: "The revolution must continue until all its objectives are met."

He added: "I promise you that I will not give up on any of the powers given to the president" - a veiled reference to recent decrees by the ruling Supreme Council of the Armed Forces (SCAF).

Morsi is to be sworn in officially on Saturday by the constitutional court, rather than by parliament as is usual.

The court dissolved the Islamist-dominated lower house this month in a series of measures designed to ensure that the generals who took over from ousted ruler Hosni Mubarak will keep a strong grip on Egypt's affairs even after Morsi takes power.

"There is no power above people power," said Morsi. "Today you are the source of this power. You give this power to whoever you want and you withhold it from whoever you want."

His defiant speech was a clear challenge to the army, which also says it represents the will of the people.

The 60-year-old U.S.-trained engineer addressed himself to "the Muslims and Christians of Egypt" and promised them a "civil, nationalist, constitutional state".

Morsi also paid homage to a militant Egyptian cleric jailed in the United States. "I see the family of Omar Abdel-Rahman (in Tahrir)," he said. "And I see the banners of the families of those who have been jailed by the (Egyptian) military." He pledged to work for the release of the prisoners, including Abdel-Rahman.

Tens of thousands of Egyptians cheered Morsi's arrival in the square that was the hub of the anti-Mubarak uprising.

"Say it loud, Egyptians, Morsi is the president of the republic," they chanted. "A full revolution or nothing. Down, down with military rule. We, the people, are the red line."

The military council that pushed Mubarak aside on Feb. 11, 2011 has supervised a chaotic stop-go transition since then, holding parliamentary and presidential elections, but then effectively negating their outcome to preserve its own power.

"Do we accept that parliament is dissolved?" cheerleaders from the Brotherhood's Freedom and Justice Party (FJP) asked the throng in Tahrir. "No," the party faithful thundered back.

Morsi was declared president last Sunday, a nerve-racking week after a run-off vote in which he narrowly beat former air force chief Ahmed Shafik, who was Mubarak's last prime minister.

After being sworn in as the first freely elected civilian president of the most populous Arab state on Saturday, Morsi would speak at Cairo University, a presidency statement said.

Hundreds of protesters have been camped out in Tahrir for weeks to press the army to transfer power to civilians.

"I'm here to tell the military council that we, the people, elected parliament so it is only us, the people, who can dissolve it," said Intissar al-Sakka, a teacher and FJP member.

She, like many of the women in Tahrir, was wearing a waist-length "khemar" veil of the kind favoured by Morsi's wife.

The military council has long promised to hand over power to the next president by July 1, but army sources said the ceremony had been postponed, without giving a reason or a new date.

The generals have seized new powers this month, giving themselves veto rights over the drafting of a new constitution, naming a National Defence Council to run defence and foreign policies and decreeing their control of all military affairs.

The military's insistence that Morsi take his oath before the constitutional court and his defiant riposte in Tahrir sets the stage for a protracted struggle for power in Egypt.

Yet it will be vital to keep such tensions in check if Egypt is to overcome economic woes that have seen foreign reserves drop by more than half in the turmoil since Mubarak's fall.

The International Monetary Fund has made a possible $3.2 billion loan conditional on broad political support for the fiscal discipline it would demand.

The Muslim Brotherhood knows it must focus on the economy to stay popular with voters, who gave it much less support in the presidential poll than in the earlier parliamentary election.

Scenes at the presidential palace occupied by Mubarak for three decades encapsulated the rise of an 84-year-old Islamist movement he had banned, constrained and often persecuted.

Bearded men, some in white robes, others in suits, milled around the palace while Morsi held talks on Thursday with the Muslim Brotherhood's supreme guide Mohamed Badie and consulted clerics from the al-Azhar seat of Islamic learning, hardline Salafis and independent evangelical Muslim preachers.

Many seemed dazzled by the grandeur of their surroundings or intrigued to be walking once-forbidden halls of power.

Security guards, still there from the Mubarak era, shook their heads in frank amazement at the bearded conclave.

After the Brotherhood's Badie entered the gates, one said: "Good God, these men were in prison before and wouldn't have dared walk past the compound. Look at them now."

Many Egyptians swarmed around outside, hoping to meet the homespun president-elect with grievances and petitions. Security men said it was hard to impose order because Morsi had given instructions that people should not be turned away.

After the talks, Morsi's Islamist visitors at the palace in Cairo's Heliopolis district broke a day-long fast with hundreds of takeout meals in cardboard boxes hauled in by palace guards from an army-owned local restaurant - one of the many commercial interests developed by the military over the decades.

The military, the source of every previous president in the Arab republic's 60-year history, runs business enterprises accounting for an estimated one-third of the economy.

It does not intend to jeopardise the $1.3 billion a year it receives in military aid from the United States to back Egypt's 1979 peace treaty with Israel, widely criticised by Islamists.

Morsi has said he will respect Egypt's international obligations and does not want to take the country back to war.

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