CAIRO—Two days after Egypt’s military replaced the country’s president, it sent soldiers into the streets to quell demonstrations, as a week of tensions between Islamists and the military transformed into deadly confrontations that heightened some Egyptians’ fears of civil war.

Demonstrations turned bloody Friday as hundreds of thousands of Muslim Brotherhood supporters turned out to protest this week’s military-led ouster of President Mohammed Morsi. Muslim Brotherhood officials said police opened fire on protesters in the Cairo suburb where Mr. Morsi and 12 aides were being held under house arrest, killing five people. The military denied those allegations.

Later in the day, armored personnel carriers arrived on the October 6 bridge, near Tahrir Square, to restore order after rival camps clashed with rocks, fireworks and, according to several witnesses, gunfire from automatic weapons. The street-level military intervention was a rare occurrence in more than two years of turmoil since the overthrow of Hosni Mubarak—but it echoed a dominant Egyptian theme of Mr. Mubarak’s long reign, one of a military-backed state pitted against Islamists.

Such fears were renewed late Friday as state television confirmed the arrest of Khairat El Shater, the Muslim Brotherhood’s influential second-in-command.

Earlier Friday, the Muslim Brotherhood’s leader addressed tens of thousands of supporters who gathered for “Rejection Friday” protests, urging them to continue protesting until Mr. Morsi is reinstated.

“We will protect our president Morsi with our necks. We are all willing to sacrifice our necks and souls for him,” Brotherhood General Guide Mohammed Badie told hundreds of thousands of demonstrators at Cairo’s Rabaa Mosque.

While he vowed peaceful protests, Mr. Badie also promised a standoff with the military, saying: “Your role is to protect our borders. Our role, however, is to bring back our president Morsi to his post.”

That poses a particular challenge to Egypt’s military, which has said it responded to overwhelming public will in removing Mr. Morsi.

In 29 months since Egyptians overthrew longtime Mr. Mubarak, the military has stood aside as street-level violence claimed thousands of lives.

The military’s unparalleled position in Egypt’s public life is a result, in part, of the perception that it is a dispassionate defender of the country’s broad public. The military made that face particularly visible in recent days, sending its jets flew above the fray below—flying triumphal missions that left contrails in the shape of hearts, or the colors of the Egyptian flag, above Cairo.

Egyptians faced off late Friday on a bridge leading to Cairo’s Tahrir Square, with witnesses reporting live gunfire between supporters of recently ousted president Mohammed Morsi and those who opposed his rule. Reem Abdellatif joins The News Hub from Cairo.

But in intervening directly as jilted Islamists demanded Mr. Morsi’s reinstatement, the military runs the risk of being seen as siding against Islamists and vindicating many Egyptians’ fears of an impending civil war.

On Friday evening, thousands of Morsi supporters attempted to use the October 6 bridge to reach Tahrir Square, where thousands of the former president’s opponents were holding rallies to celebrate Mr. Morsi’s ouster and “defend” the revolution.

According to witnesses, the opposing sides clashed on the bridge. Two witnesses said that Morsi opponents fired automatic weapons against the pro-Morsi arrivals, and the sound of repeating fire could be heard in the area. Shortly after, according to witnesses, the sides appeared to run out of ammunition, and turned to throwing rocks and fireworks.

Associated PressSupporters and opponents of Egypt’s ousted President Mohammed Morsi clashed in Cairo Friday.

Col. Ahmed Ali, the military’s spokesman, said the military used extreme caution late Friday as it mobilized forces to the bridge, to avoid advancing the perception that Egyptian soldiers were cracking down on Islamists.

“They are trying to pick a fight with the military, to confirm that there is a coup,” said Col. Ali, characterizing the Brotherhood as power-hungry.

“They don’t care about the Egyptian people, they just want to get back in power. I mean, come on, this is the MB—they had power before and they won’t let go of it regardless of what the Egyptian people really want.”

Later Friday, Morsi supporters, clad in helmets and carrying wooden clubs, guns, knives and rocks, were seen marching toward Maspero, Egypt’s state television building, according to Egyptian television and a witness.

Meanwhile, military forces could be seen securing the state television building and other government buildings and state ministries. Army tanks could be seen on roads surrounding Tahrir Square and other areas in Cairo, particularly in districts with embassies and consulates.

Associated PressAn Egyptian soldier removed a poster of ousted President Mohammed Morsi outside the Republican Guard building in Cairo Friday.

Egypt’s army declared a state of emergency in Suez and southern Sinai ahead of protests from the Muslim Brotherhood against the military’s overthrow of President Mohammed Morsi and the arrests of its leaders. Matt Bradley reports. Photo: AP.

The day’s first fatal violence occurred near the Republican Guard Guesthouse in Nasr City, a Cairo suburb, where Brotherhood and military officials alike say Mr. Morsi and his aides are under house arrest.

Photos: Egypt Military Launches Crackdown

See photos from the clashes in Cairo.

AFP/Getty ImagesProtesters supporting ousted President Mohammed Morsi carried a man who was shot during clashes between Egypt’s Army and the Muslim Brotherhood outside the Cairo headquarters of the Republican Guard Friday.

 

Timeline: Egypt in Transition

Track key dates over the past two years.

Associated Press

 

Checking In

Arab Spring two years later

The removal of Mohammed Morsi as Egyptian president will come as a big blow for the Muslim Brotherhood, which has been trying to gain power for decades. Charles Levinson looks at the fallout for the brotherhood both in Egypt and in other Middle East countries. Photo: AP

Associated PressSmoke in the colors of the Egyptian national flag trails from military aircraft as an opponent of Egypt’s ousted President Mohammed Morsi flashes V signs in Tahrir Square in Cairo on Friday.

Five protesters were killed in clashes with the military outside the guesthouse, said Gehad Al Haddad, a spokesman for the Muslim Brotherhood. He pointed to a video on YouTube that he said shows the military firing on peaceful protesters. Gunshots can be heard in the video, which also shows one person injured and bloodied and another image of a person gathering around what appears to be a body covered in a shroud. Its authenticity couldn’t be verified.

Mr. Al Haddad said that in at least one instance, an officer fired on a protester from about six feet away. “It was a pure assassination,” Mr. Haddad said.

Egyptian soldiers denied allegations that they had used live ammunition against Mr. Morsi’s supporters, according to an army spokesman, who said soldiers had fired blank rounds into the air as warnings to protesters.

Getty ImagesAn anti-Morsi protester is attended to after allegedly being shot by Muslim Brotherhood supporters in Tahrir Square Friday.

“When they failed to enter the Republican Guard headquarters, pro-Morsi protesters hurled Molotovs at the military,” said Col. Ali, the military’s spokesman. “The military, police and Republican Guard responded with rounds of tear gas to push back the protesters.”

The Ministry of Health reported that two people were killed and 65 were wounded in fighting in front of the Republican Guard club.

Meanwhile, interim President Adly Mansour issued a presidential declaration dissolving the Shura Council, the Islamist-dominated legislative body that has managed most legislative duties since courts dissolved the parliament’s lower house last summer, according to Egyptian state television.

Mr. Mansour also appointed a member of Mr. Morsi’s government, Rafa’at Shehatta, as an adviser to the president for security affairs. Mr. Morsi appointed Mr. Shehatta as head of national intelligence last September. Mr. Shehatta was involved in negotiations leading to the October 2011 release of Gilad Shalit, the Israeli soldier held for more than five years by Hamas.

Mr. Mansour also appointed a new head of general intelligence, Mohammed Farid Al Tohami, according to state television.

The African Union said it has suspended Egypt’s membership following the “unconstitutional” ouster of Mr. Morsi.

U.K. oil giant BP BP.LN -0.16% PLC, the key oil and gas producer in Egypt, said Friday it is pulling some staff from Egypt as “a precautionary measure,” indicating the nation’s political crisis is starting to affect one of the country’s most profitable industries.

Violence unfolded early in the day after a protester held a picture of Mr. Morsi up against the barbed wire surrounding the Republican Guard club, said Mahmoud Younes, a Brotherhood supporter there. A plainclothes military officer grabbed the picture and tore it up, said Mr. Younes.

Protesters then threw water bottles toward the military officers, who responded by firing tear gas.

One military officer, speaking through a loudspeaker, pleaded with protesters. “Let’s not provoke one another at a time like this, not when there’s been death,” the officer said with an even cadence. “These soldiers are your brothers—don’t provoke them.”

The protesters stood their ground and chanted “peaceful, peaceful.”

Two Apache helicopters appeared in the sky behind the soldiers. “Hold your ground,” several protesters yelled. Then came the sound of several dull pops, followed by a trail of tear gas that caused protesters to scatter wildly.

Pro-Morsi protesters were seen fainting from tear-gas inhalation. Badly injured men could be seen carried out of the crowd with faces and limbs bloodied from what appeared to be birdshot.

One man was unconscious and had a gaping wound in his forehead. It was unclear if it had been caused by live fire.

Following the violence, hundreds of pro-Morsi marchers rushed toward the Republican Guard building.

“We were peaceful until the final moments,” said Abdel Fattah Awadallah, 35, a project manager who was holding up his bloodied palms to passing traffic. The blood, he said, came from a shooting victim.

By nightfall, a flatbed truck with massive speakers occupied the roadway, surrounded by thousands of demonstrators who sat on the asphalt and listened to anti-military slogans and speeches. As the numbers grew larger, the sit-it crept closer to the barricaded headquarters. The soldiers on the other side of the barbed wire stood sentry, quietly.