World powers said Wednesday that the transition towards democracy in Egypt must begin immediately and called for calm amid clashes in Cairo after Hosni Mubarak vowed to cling to office until September.

U.N. Secretary General Ban Ki-Moon called attacks on peaceful protestors "unacceptable" as pro- and anti-Mubarak demonstrators fought each other in the city's central Tahrir Square, leaving at least 500 injured.

Supporters of President Hosni Mubarak stormed a crowded anti-regime rally in central Cairo on Wednesday, sparking pandemonium in which at least 500 people were hurt and one killed, witnesses said.

Partisans from both sides threw stones and set on each other with sticks and fists in battles that raged for hours in Tahrir Square, the epicenter of anti-regime demonstrations, a day after Mubarak vowed to see out the rest of his term.

The Egyptian military called Wednesday for an end to more than a week of demonstrations against President Hosni Mubarak, throwing its support behind his embattled regime hours after he defiantly rejected demands to step down immediately and said he would serve out his term in office.

The declaration was a clear shift in the stand of the army, which gave a tacit endorsement to the movement on Monday by saying it would not use force against protesters, and that they had legitimate demands.

The United States is conducting a manhunt for a previously unknown group believed to be involved in the planning of the 9/11 attacks, according to a US cable published in Wednesday's Telegraph newspaper.

In the memo, leaked by the WikiLeaks website, a US official in Qatar told the Department for Homeland Security in Washington that three Qatari men were under suspicion of conducting surveillance operations on the attack sites.

Yemen's president has told parliament he will not seek another term in office or hand power to his son — an apparent reaction to protests in his own country that have been inspired by Tunisia's revolt and the turmoil in Egypt.

The U.S.-allied Ali Abdullah Saleh, who has been in power for nearly 32 years, spoke Wednesday to lawmakers in both houses of the assembly.

President Barack Obama on Tuesday challenged Egypt's embattled autocratic ruler, a staunch U.S. ally, to begin immediately the process of transitioning the country to new leadership, a signal that there should be no drawn-out goodbye.

Earlier, Egyptian President Hosni Mubarak had announced he would not seek another term in office but also would not yield to growing demands to step down now. After a huddle at the White House and a 30-minute telephone conversation with Mubarak, Obama went on television to respond.

Syrians are organizing campaigns on Facebook and Twitter that call for a "day of rage" in Damascus this week, taking inspiration from Egypt and Tunisia in using social networking sites to rally their followers for sweeping political reforms.

Like Egypt and Tunisia, Syria suffers from corruption, poverty and unemployment. All three nations have seen subsidy cuts on staples like bread and oil. Syria's authoritarian president has resisted calls for political freedoms and jailed critics of his regime.

Egyptian President Hosni Mubarak pledged on Tuesday that he would not stand for re-election in September, in an address to the nation that came after eight days of anti-government demonstrations.

"In all sincerity, regardless of the current circumstances, I never intended to be a candidate for another term," he said.

U.S. President Barack Obama has told his beleaguered Egyptian counterpart Hosni Mubarak not to stand for re-election in upcoming September balloting, the New York Times said Tuesday.

The message from Obama was conveyed by former U.S. ambassador to Egypt, Frank Wisner, who was dispatched to Cairo on Monday by the U.S. administration, the influential U.S. daily said, quoting American diplomats.

France urged Egypt on Tuesday to end the "bloodshed" that has left as many as 300 people dead in anti-government protests there.

"That must stop," French foreign ministry spokesman Bernard Valero told reporters. "The blood must stop flowing. There have been too many deaths, too many wounded."