WRAPUP 15-Mubarak Backers Attack Cairo Protesters, 3 Dead

* Health ministry says three dead, doctor says 1,500 injured

* Egyptian army tells protesters to clear the streets

* Mubarak says he will step down in September

(Raises death toll, updates situation in Tahrir square)

By Shaimaa Fayed and Yasmine Saleh

CAIRO, Feb 2 (Reuters) - Backers of Egyptian President Hosni Mubarak, throwing petrol bombs, wielding sticks and charging on horses and camels, assaulted demonstrators in Cairo on Wednesday after the army told the protesters to go home.

Anti-Mubarak protesters hurled stones back and said the attackers were police in plainclothes. The Interior Ministry denied the accusation, and the Egyptian government rejected international calls for Mubarak to end his 30-year rule now.

This apparent rebuff along with the appearance of Mubarak supporters on Cairo's streets and their clashes with protesters -- after days of relatively calm demonstrations -- complicated U.S. calculations for an orderly transition of power.

In pointed comments, a senior U.S. official said it was clear that "somebody loyal to Mubarak has unleashed these guys to try to intimidate the protesters".

The emergence of Mubarak loyalists, whether ordinary citizens or police, injected a new dynamic into the momentous events in this most populous Arab nation of 80 million people.

The uprising broke out last week as public frustration with corruption, oppression and economic hardship under Mubarak boiled over. At least 140 people are estimated to have been killed so far and there have been protests across the country.

As night fell, Egyptian Vice President Omar Suleiman urged the 2,000 demonstrators bedding down in Cairo's central Tahrir (Liberation) Square to leave and observe a curfew to restore calm. Suleiman said the start of dialogue with the opposition depended on an end to street protests.

After dark the protesters barricaded the square against groups of pro-Mubarak supporters who appeared to be trying to penetrate the makeshift cordon. There was sporadic gunfire, with blazes caused by firebombs, and the atmosphere was tense.

Officials said three people were killed in Wednesday's violence and a doctor at the scene said over 1,500 were injured.

Reacting to the tumult in Egypt, a key ally, the White House said it was vital for clashes to stop to ease a power handover. "If any of the violence is instigated by the government it should stop immediately," spokesman Robert Gibbs said.

Opposition figurehead Mohamed ElBaradei, a Nobel Peace laureate, called on the army to intervene to stop the violence in Tahrir Square, the worst in the nine-day uprising against Mubarak since protesters waged street battles last Friday.

Troops and tanks stood by as the violence raged.

Urging protesters to clear the streets, the armed forces told them their demands had been heard. But some were determined to occupy the square until Mubarak quits.

Khalil, a man in his 60s holding a stick, blamed Mubarak supporters and undercover security men for the clashes.

"We will not leave," he told Reuters. "Everybody stay put."

U.S. CONFLICTED

The crisis has alarmed the United States and other Western governments who have regarded Mubarak as a bulwark of stability in a volatile region, and has raised the prospect of unrest spreading to other authoritarian Arab states.

Mubarak went on television on Tuesday to say he would not stand in elections scheduled for September. This was not good enough for the protesters, who demanded he leave the country.

President Barack Obama telephoned the 82-year-old to say Washington wanted him to move faster on political transition.

"The message that the president delivered clearly to President Mubarak was that the time for change has come," Gibbs said, adding: "Now means now." Secretary of State Hillary Clinton, in a call to Suleiman, underlined that U.S. position.

But Mubarak dug in his heels on Wednesday. A Foreign Ministry statement rejected U.S. and European calls for the transition to start immediately, saying they aimed to "incite the internal situation" in Egypt.

"This appears to be a clear rebuff to the Obama administration and to the international community's efforts to try to help manage a peaceful transition from Mubarak to a new, democratic Egypt," said Robert Danin, a former senior U.S. official now at the Council on Foreign Relations think tank.

The administration will want to see order restored without compromising the standing of the Egyptian army, which it supplies annually with about $1.3 billion in aid.

Many analysts see the army as trying to ensure a transition of power that would allow it to retain much of its influence.

International backing for Mubarak, for three decades a stalwart of the West's Middle East policy, a key player in the Middle East peace process and defence against the spread of militant Islam, crumbled as he tried to ride out the crisis.

France, Germany and Britain also urged a speedy transition.

Some of the few words of encouragement for him have come from oil giant Saudi Arabia, a country seen by many analysts as vulnerable to a similar outbreak of discontent.

Israel, which signed a peace treaty with Egypt in 1979, is also watching the situation in its western neighbour nervously, weighing the possibility that Islamists hostile to the Jewish state might gain a share of power in Cairo.

Prime Minister Benjamin Netanyahu called on Wednesday for "bolstering Israel's might" in response to turmoil n Egypt.

FIGHTING IN THE SQUARE

Troops made no attempt to intervene as opposing factions clashed in the vast Tahrir Square, the focus of the protests. Attackers brandished baseball bats and iron bars and broke up pieces of paving stones to throw.

Earlier, pro-Mubarak youths were bussed into various districts of the capital. Thousands were involved in what escalated into pitch battles.

Reuters correspondents saw dozens injured and people fleeing in panic. One of the riders who wielded whips and sticks as they galloped into the crowd was dragged from his horse and beaten.

Petrol bombs landed in the gardens of the Egyptian Museum. Journalists said they were targeted by pro-Mubarak supporters.

An opposition coalition, which includes the Islamist Muslim Brotherhood group, responded to the army warning to leave Tahrir Square by calling for more protests. It said it would only negotiate with Suleiman, a former intelligence chief appointed by Mubarak at the weekend, once the president stepped down.

At the weekend, Mubarak reshuffled his cabinet and promised reform but that was not enough for the protesters.

One million people took to the streets of Egyptian cities on Tuesday calling for him to quit. Many protesters spoke of a new push on Friday, the Egyptian weekend, to rally at Cairo's presidential palace to dislodge Mubarak.

The uprising was inspired in part by a popular revolt in Tunisia last month which overthrew long-ruling President Zine al-Abidine Ben Ali. The mood is spreading across the region.

King Abdullah of Jordan replaced his prime minister on Tuesday following protests there. Yemeni President Ali Abdullah Saleh, an important U.S. ally in the fight against al Qaeda, said on Wednesday he would not seek to extend his presidency.

Oil prices fell back from 28-month highs, but North Sea Brent crude was still more than $101 a barrel because of worries that unrest in Egypt could kindle yet more political upheaval across the Middle East and North Africa.

But with Mubarak pledging to go, foreign investors have begun to show renewed interest in Egyptian bonds and stocks, and the cost of insuring Egyptian debt against default fell. (Additional reporting by Andrew Hammond, Patrick Werr, Dina Zayed, Marwa Awad, Shaimaa Fayed, Alexander Dziadosz, Yasmine Saleh, and Alison Williams in Cairo; Writing by Peter Millership; editing by Mark Heinrich)