CAIRO (Reuters) - Egypt's President-elect Mohamed Mursi takes his oath on Saturday, a day after the Islamist leader pre-empted the formal ceremony by swearing himself in before ecstatic crowds in Tahrir Square and warning off generals trying to curb his powers.

They have already clipped the prerogatives of the bearded leader now in the palace once occupied by Hosni Mubarak, who is serving a life sentence 16 months after Egyptians toppled him.

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

Addressing the "Muslims and Christians of Egypt", he promised a "civil, nationalist, constitutional state", making no mention of the Brotherhood's dream of creating an Islamic order.

Mursi is to be sworn in officially at 11 a.m. (05.00 a.m. EDT) by the constitutional court, rather than by parliament as is usual.

The court dissolved the Islamist-dominated lower house this month in one of several measures intended to entrench military influence over Egypt long after Mursi assumes the presidency.

"There is no power above people power," Mursi said in Tahrir Square, crucible of the revolt that ended Mubarak's 30 years in power. "Today you are the source of this power."

His defiant speech was a clear challenge to the army, which also says it embodies the will of the people and which sees itself as the guarantor of national interests and the state.

An army council headed by Field Marshal Mohamed Hussein Tantawi took over from Mubarak when he quit on February 11, 2011 and has long pledged to make way for an elected president by July 1.

Tantawi, Mubarak's defense minister for 20 years, would hand over to Mursi in a televised ceremony on Saturday after the president was sworn in, army sources said.

Under the military council's rule, Egypt has experienced a bumpy and sometimes violent transition in which parliamentary and presidential elections have been held, without setting the country on a clear path to democracy or constitutional rule.

LIVING IN LIMBO

Egypt remains in political limbo, without a constitution, a lower house of parliament or any clarity about the role of a military establishment anxious to stay in the driving seat.

An assembly that is supposed to write a new constitution has begun work after its predecessor fell apart amid disputes over whether Islamists were over-represented in a country with a 10 percent Christian minority and many secular-minded liberals.

Egypt is also more polarized than ever.

Mursi narrowly won a run-off vote this month against Ahmed Shafik, a former air force chief and Mubarak's last prime minister, but many voters were dismayed at having to choose between an Islamist and a man seen as a remnant of Mubarak's era.

Egypt will find it hard to attract the investment, loans and foreign aid it needs to revive an economy blighted by months of turmoil and uncertainty until political stability returns.

International Monetary Fund Managing Director Christine Lagarde called Mursi to discuss the economic challenges facing Egypt and how the international lender can best help, an IMF spokesperson said on Friday.

"The MD reiterated that the IMF stands ready to support Egypt and looks forward to working closely with the authorities," the spokesperson said.

Lagarde also congratulated Mursi on his election as president, "which represents an important step forward in Egypt's transition," the spokesperson said.

However, no timetable has been set for an IMF staff visit to Egypt to discuss a $3.2 billion IMF loan. That "will depend on the formation of the government," the spokesperson said.

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

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

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

It does not intend to jeopardize 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 criticized by Islamists.

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

His speech in Tahrir, greeted with cheers from tens of thousands of supporters, signaled a determination to use popular legitimacy to defeat entrenched military power.

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

(Additional reporting by Marwa Awad, Shaimaa Fayed and Omar Fahmy; Editing by Ralph Gowling and Eric Walsh)

GENEVA (Reuters) - International talks on a way to resolve the increasingly bloody conflict in Syria opened in Geneva on Saturday with foreign powers in dispute over the fate of President Bashar al-Assad.

Kofi Annan, the former U.N. chief who is special international envoy on Syria, has been hoping for consensus on a plan for a unity government that, by excluding from the leadership figures deemed too divisive, would effectively mean Assad stepping down.

However, Moscow, a long-time ally of the Syrian strongman and an opponent in principle of what it sees as foreign meddling in domestic sovereignty, has voiced objections to any solution imposed on Syria from outside.

The United States and its European and Arab allies see no way ahead while power remains in the hands of Assad. The U.N. estimates at least 10,000 people have been killed as Assad's forces have tried to suppress the uprising against him.

"It has always been our view that a stable future for Syria, a stable political process means Assad leaving power as part of an agreement on transitional process," British Foreign Minister William Hague said in Geneva.

Russian Foreign Minister Sergei Lavrov said nothing to reporters at the talks.

He and U.S. Secretary of State Hillary Clinton Russia met in St.Petersburg, Russia, on Friday night but a U.S. official said differences with Moscow over the conflict remained.

"Our Western partners want themselves to decide the outcome of the political process in Syria although it is the job for the Syrians," Lavrov's Deputy Gennady Gatilov said in Russia prior to the Geneva meeting.

Clinton offered no further insights as she arrived for the talks. But Britain's Hague made clear he expected a day of hard bargaining.

"There is an opportunity for the international community to be much stronger and act more robustly but we can only do it with the agreement of Russia and China," he said.

The foreign ministers of the five permanent members of the U.N. Security Council - Russia, the United States, China, France and Britain - were attending Saturday's talks.

Turkey, Kuwait, Qatar, U.N. Secretary-General Ban Ki-moon and Arab League chief Nabil Elaraby are also taking part.

However, Iran, Syria's closest regional ally, and Saudi Arabia, a foe of both Damascus and Tehran and leading backer of the rebel forces opposing Assad, are not represented. Nor is anyone from the Syrian government or opposition.

WORSENING BLOODSHED

The Syrian conflict has gone from bad to worse since the uprising broke out 16 months ago, evolving from peaceful protests against the Assad family's four-decade dynastic rule to something akin to a civil war with a sectarian dimension.

Although the world has condemned the ferocity of Assad's forces' crackdown on the opposition - including bombardments of pro-opposition areas and mass arrests - it has been unable to halt violence which threatens to draw in more of the region's religious and political rivalries and alliances.

Annan, who brokered a much abused ceasefire in April, said on Friday he was "optimistic" that the Geneva talks would produce an acceptable outcome. Later, senior officials holding preparatory talks there failed to overcome differences. Western diplomats said Russia was pressing for changes to Annan's text.

"It is absolutely essential that the violence stops and that a political transition can begin. Kofi Annan made reasonable propositions and I hope that they will be upheld and that's the point of today's discussions," French Foreign Minister Laurent Fabius, said as he arrived for the talks.

An Arab diplomat told Reuters on Friday night after a preliminary meeting ended in Geneva that the state of the plan was looking bad.

"If there is no agreement, Bashar al-Assad will know he had every possible opportunity to fly his planes and burn towns and the international community will do nothing," he said.

Russia, and China, both conscious of the risk of internal revolt at home, have objected to what they see as Western interference in the domestic affairs of rulers like Libya's Muammar Gaddafi.

Western governments, however, have shown little will to repeat last year's Libyan experience of military support for Arab rebels in Syria, where Assad's forces are formidable and the complexities of religion and ethnicity much greater.

On the ground in Syria, fighting continued on Friday, with particular tension around the northern border with Turkey, a week after Syria shot down a Turkish warplane.

Syrian helicopter gunships bombarded a strategic town in the north and tanks moved close to the commercial hub of Aleppo, rebels said. But Syrian troops kept well clear of new Turkish air defenses installed to curb Syrian action near its borders.

(Additional reporting by Andrew Quinn and Liza Dobkina in St. Petersburg, Tim Heritage and Gleb Bryanski in Moscow and Khaled Yacoub Oweis in Antakya, Turkey; Writing by Angus MacSwan; Editing by Matthew Tostevin)