By Alexander Dziadosz and Shadia Nasralla

CAIRO, July 9 (Reuters) - When 55 people protesting against the military overthrow of Egypt's first freely elected president were killed after the army opened fire on Monday, you might have expected the country to unite in condemnation.

A surprisingly subdued public reaction, and the independent media's outright vilification of protesters, reflects in part the depth of political opponents' distrust of Mohamed Mursi and his Muslim Brotherhood.

But it also represents a triumph for the military's public relations machine which, aware of its fumbled handling of the turbulent aftermath of Hosni Mubarak's overthrow in 2011, has moved decisively, and successfully, to gain the upper hand.

By contrast, the Muslim Brotherhood and its supporters, enraged by the killings, have struggled to make their voice heard since the army overthrew Mursi last Wednesday.

The state-owned media have reprised their role as faithful mouthpiece of military-backed authorities since then. Perhaps unsurprisingly, they have given full-throated support to the army version that the pro-Mursi protesters attacked the Republican Guard compound and soldiers fired in self defence.

"Terrorists try to break into the Republican Guard," was the banner headline on the front page of state-run Al-Gomhuria newspaper on Tuesday morning, referring to the barracks where Mursi is believed to be held and where the killings occured.

Al-Akhbar, another state-owned paper, splashed front page pictures of soldiers carrying a wounded comrade and an image purportedly showing a protester firing on soldiers.

Independent newspapers, many of which were fiercely opposed to Mursi when he was in power, have been, if anything, more partisan. Daily Al-Masry Al-Youm wrote the bloodletting was "the Brotherhood's responsibility." Al-Watan decried a "conspiracy by the 'Armed Brotherhood' against the army."

The military says armed men attacked troops in the area around the Republican Guard compound early on Monday morning, and that one officer was killed in the clashes and 42 were wounded, eight of them critically.

DOUBT AND BLAME

With television stations sympathetic to the Brotherhood shut down, senior leaders arrested and its newspaper appearing only intermittently, Mursi's supporters have struggled to convey their view of the killings - that security forces, unprovoked, fired on them while they conducted dawn prayers.

"The military coup has showed its hideous face after just six days," said a flyer handed out by young men at the main pro-Mursi sit-in at a mosque in northeastern Cairo.

"Were these people firing bullets while they bowed upon their mats in prayer?"

It is impossible to tell exactly where the weight of opinion lies among Egyptians, but the army's bid to marginalise and isolate the Brotherhood - its longtime rival for influence in public life - resonates on the streets of Cairo.

"It's horrible to spill blood. But, of course, the Brotherhood are terrorists, attacking the Republican Guard," one woman who works in a bank said, while doing her morning shopping in downtown Cairo.

"The Brotherhood want to get Mursi back by all means, even violence. The people want the opposite, not terrorism," Ibram Rez, 22, a tourism management graduate, said.

Abdallah Abdel Rayal, 58, a clothes and cleaning product company employee, said he condemned the bloodshed but insisted "the people attacked the army, not the other way around."

As he spoke, a woman walking past shouted: "The people attacked the Guard! May God take them!"

Heba Morayef, Egypt director at Human Rights Watch, said the army had improved its public relations machine markedly since the tumultuous 17 months the military spent running the country after Mubarak's fall.

At that time, many people blamed the army for violent crackdowns on protests and activists, which led the military to make several ill-judged responses.

This time, a new army spokesman - the urbane, British-trained Colonel Ahmed Ali - called a press conference to make the military's case plainly and clearly, using videos taken during the clashes to try to prove his point.

Journalists applauded when he finished.

"They weren't under any public pressure, and they knew there wouldn't be any push back," Morayef said.

"THE SAME CRACKDOWN"

While Mursi's supporters call his overthrow a coup, the military says it was merely enforcing the will of the people after millions rallied on June 30 to call for his resignation.

At a heated press conference on Monday, supporters of Mursi dumped plastic bags of bullet casings and teargas canisters on a table in front of television cameras and brought a doctor from the protest field hospital to contest the army's claims.

Brotherhood spokesman Gehad El-Haddad said the killing at the Republican Guard barracks showed Egypt's police state had survived unaltered since the uprising against Mubarak.

"It's the same mentality, it never changes - the same military junta, the same guys on TV, the same crackdown," he told Reuters, his voice cracking with emotion. But even Haddad acknowledged the group's new limits.

"When this is over, they will all be put on trial, every conspirator to that coup," he said, and then paused.

"Or should be, you might say, given that we're the opposition now." (Writing by Alexander Dziadosz; editing by Mike Collett-White and Ralph Boulton)