* Egypt's military aloof from protest drama so far
* Armed forces at heart of power for 60 years
* Military "ready to intervene" - security source
By Alistair Lyon, Special Correspondent
BEIRUT, Jan 28 (Reuters) - Egypt's military has kept out of this week's clashes between police and protesters demanding the ousting of 82-year-old President Hosni Mubarak, but it could eventually decide his fate, echoing events in Tunisia.
A Tunisian army general's refusal to back Zine al-Abidine Ben Ali's crackdown on protesters is widely regarded as a turning point that forced the former president to quit Tunisia on Jan. 14 after weeks of popular protests.
Egypt's military might not react in the same way, but after watching the interior ministry's police and security forces struggle to contain four days of unprecedented street protests, the generals may well be considering their options.
"Indicators confirmed the Egyptian armed forces are ready to intervene in Suez and other parts of Egypt if necessary," said a security source in Cairo on Thursday, refusing to be named.
Egypt's sprawling armed forces -- the world's 10th biggest and more than 468,000-strong -- have been at the heart of power since army officers staged the 1952 overthrow of the monarchy.
All four Egyptian presidents since then have come from the military, now led by Field Marshal Mohamed Hussein Tantawi, 75, who is defence minister and commander-in-chief. It benefits from about $1.3 billion a year in U.S. military aid.
The armed forces are eclipsed numerically by the internal security forces under Interior Minister Habib el-Adli, 72. These have grown since the failed Islamist revolt of the 1990s into a vast force of 1.4 million, say U.S. diplomats in leaked cables.
The military and internal security, along with a ruling party machine and an emerging business elite, form the core of an establishment that has sustained Mubarak's 30-year rule.
Could the fierce surge of popular unrest drive cracks in this apparatus that has underpinned his authoritarian rule?
The military is notoriously opaque. Reporting on it remains taboo, even in the much more vibrant media scene that has blossomed in Egypt in recent years. Little is known about its substantial land holdings, huge economic interests or budget.
"The idea that the military remains a key political and economic force is conventional wisdom here," said a U.S. diplomatic cable from July 2009 released by WikiLeaks on Friday.
"However, other observers tell us that the military has grown less influential, more fractured and its leadership weaker in recent years," the cable from the U.S. embassy in Cairo said.
Nevertheless, a source with insight into how some officers are thinking scorned the "sloppy" performance of the police and security forces against the protests of the last few days and suggested the military would step in if Egypt fell into chaos.
"The army has delayed its response, like in Tunisia," said the source, who did not want to be identified.
He said the military's priority was external defence, not police work, but he outlined a scenario in which the army would act decisively to quell unrest, impose curfews and perhaps take sweeping decisions on Egypt's political future.
Such a scenario, which would have seemed unlikely just weeks ago, looks less far-fetched given Ben Ali's overthrow and the tremors of anger shaking Egypt, Yemen and other Arab states.
A research note by Japanese bank Nomura takes a more measured view: "Although we do not rule out the possibility of the military in Egypt looking to engineer an orderly leadership transition if needed, we doubt that it would deliberately act in a manner which risked overturning the regime," it said.
Hossam Hamalawy, an Egyptian opposition activist, speaking before the unrest began, said army intervention would lead to a blood-bath or a refusal by troops to fire on their compatriots.
"Soldiers are used to war and fighting foreign enemies ... unlike police who are engaged in a daily repression job against the citizens," he said, although he added that ordinary policemen share the economic grievances of society at large.
"In the case of the army, it is even worse. The army cannot stand a confrontation with the Egyptian people today. If an uprising takes place, and the army gets sent in, I expect a disaster for the regime, not for the people," Hamalawy said.
The army was used to quell bread riots in Egypt in 1977 and to halt a rampage by policemen over pay in 1986.
Until recently, most speculation about Egypt's military centred on its attitude to the presidential succession.
Several leaked U.S. cables argue that the military could obstruct any attempt to install Gamal Mubarak, a businessman and politician with no military background, to replace his father should the latter not run in a presidential election scheduled for September.
However, the July 2009 cable from the Cairo embassy quoted a ruling party insider and former minister, Ali El Deen Hilal Dessouki, as saying the Egyptian military and security services would ensure a smooth transfer of power, "even to a civilian".
President Mubarak has not said if he will stand for a sixth term. Both he and Gamal deny any plans for a family succession.
But the tens of thousands of Egyptians demonstrating across Egypt on Friday are bent on making all the speculation academic.
"Leave, leave, Mubarak, Mubarak, the plane awaits you," protesters shouted outside a Giza mosque after Friday prayers. (Editing by Tim Pearce)