Khaled el-Khatib, a Health Ministry official, says four people were killed near the Republican Guard building in Cairo, where troops opened fire on Morsi supporters marching on the building Friday afternoon.
In Cairo, another person was killed in clashes that erupted after nightfall when Islamists attacked Morsiopponents near Tahrir Square.
Four others died in the northern Sinai city of el-Arish, where Islamists stormed the main government building. The 10th was killed in the southern city of Assiut.
El-Khatib says 210 people nationwide have been wounded.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
Enraged Islamists pushed back against the toppling of President Mohammed Morsi, with tens of thousands of his supporters marching in Cairo on Friday to demand the reinstatement of Egypt's first democratically elected leader. Soldiers fired on protesters, crowds of Islamists descended on Morsi opponents in stone-throwing and gun-firing clashes, and armored vehicles deployed on bridges over the Nile in mayhem that left at least six dead.
In a dramatic appearance — his first since Morsi's ouster — the supreme leader of the Muslim Brotherhood defiantly vowed the president would return. "God make Morsi victorious and bring him back to the palace," Mohammed Badie proclaimed from a stage before a crowd of cheering supporters at a Cairo mosque. "We are his soldiers we defend him with our lives."
Badie addressed the military, saying it was a matter of honor for it to abide by its pledge of loyalty to the president, in what appeared to be an attempt to pull it away from its leadership that removed Morsi. "Your leader is Morsi ... Return to the people of Egypt," he said. "Your bullets are not to be fired on your sons and your own people."
After nightfall, moments after Badie's speech, a large crowed of Islamists surged across 6th October Bridge over the Nile toward Tahrir Square, where a giant crowd of Morsi's opponents had been massed all day. Battles broke out there at near the neighboring state TV building with gunfire and stone throwing.
A fire burned on the bridge as Islamists sporting makeshift shields and wearing helmets they had brought in preparation traded stones with their opponents.
"They are firing at us, sons of dogs, where is the army," one Morsi opponent shouted, as another was brought to medics with his jeans soaked in blood from wounds in his legs. Army troops deployed on another Nile bridge leading into Tahrir, sealing it off with barbed wire and armored vehicles.
In cities across the country, clashes erupted as Morsi supporters marched on local government buildings, battling police or Morsi opponents. At least six people were killed throughout the day — four of them in Cairo, with at least 180 wounded, Health Ministry official Khaled el-Khatib told The Associated Press.
"We are all now afraid for Egypt," Amr Moussa, a former Arab League chief who was a major leader of the opposition to Morsi during his year in office, said on Al-Hayat TV. "Egypt can't afford to enter into violence or civil war."
But Islamists vowed to show by their numbers and the turmoil that the military had made a mistake in oustingMorsi on Wednesday night after millions of Egyptians poured into streets around the country for four days this week demanding the Islamist president go in the biggest rallies the country has seen.
Badie's speech injected a new vehemence into Morsi's supporters, and the eruption of clashes at multiple locations soon afterward suggested a coordinated counter-push against those behind his ouster.
"The military got itself in a trap by siding by one side. Now they see the masses in the streets and now they realized that there are two peoples," Hamada Nassar, a figure from the hard-line former militant group, Gamaa Islamiya, told AP.
As clashes raged Friday night, the military's spokesman warned against "any provocation or contact with groups of peaceful protests, and those who transgress that will be dealt with complete determination according to the law."
The day's turmoil began in the afternoon when army troops opened fire as hundreds of Morsi supporters marched on the Republican Guard building in Cairo, where Morsi was staying at the time of his ouster before being taken into military custody at an unknown location.
The crowd approached a barbed wire barrier where troops were standing guard around the building. When one person hung a sign of Morsi on the barrier, the troops tore it down and told the crowd to stay back. A protester put up a second sign, and the soldiers opened fire, according to an Associated Press photographer.
One protester was killed, with a gaping, bleeding wound in the back of his head, while others fell bloodied and wounded. Witnesses told to AP Television News at the scene that men in plainclothes fired the lethal shots.
Protesters pelted the line of troops with stones, and the soldiers responded with volleys of tear gas. Many of those injured had the pockmark wounds typical of birdshot. The BBC's Middle East editor, Jeremy Bowen, was hit by birdshot in the head as he covered the clashes. "Am fine," he reported in a tweet.
Three hours later, Badie — who security officials had previously said was taken into custody soon after Morsi'sremoval — made his appearance before tens of thousands of Islamists massed at Rabia al-Adawiya Mosque, not far from the Republican Guard building.
Morsi "is my president and your president and the president of all Egyptians," Badie proclaimed, thrusting his arms in the air, as a military helicopter circled low overhead.
The grey-haired Badie, the group's "general guide," is a revered figure among the Brotherhood's followers, who swear an oath of absolute obedience to him — to "hear and obey."
The circumstances of his appearance were a mystery, however. Security officials had said Badie was taken into custody Wednesday night from a villa on the Mediterranean coast and flown to Cairo, part of a sweep that netted at least five other senior Brotherhood figures and put around 200 more on wanted lists.
Just before Badie's appearance, the Brotherhood's political party said on its webpage that he had "been released." But on stage, Badie denied he was ever arrested. There was no immediate explanation by security officials for the circumstances of his detention and release.
Authorities also announced the release of Saad Katatni, head of the Brotherhood's political arm the Freedom and Justice Party, as well as one of Badie's deputies, Rashad Bayoumi, pending further investigation.
Britain's Foreign Secretary William Hague said he was "very concerned" by the reports of violence. In a Twitter message, he wrote: "Hope calm heads will prevail, vital to avoid escalation."
Fears have been high over a major Islamist backlash to the military's move. The Brotherhood has said it will not work with the new military-backed leadership. Morsi supporters say the military has wrecked Egypt's democracy by carrying out a coup against an elected president. They accuse Mubarak loyalists and liberal and secular opposition parties of turning to the army for help because they lost at the polls to Islamists. Many supporters have equally seen it as a conspiracy against Islam.
Extremist Islamist groups that gained considerable freedom to operate during Morsi's year in office have already vowed violence in retaliation.
The first major Islamic militant attack came before dawn Friday in the tumultuous Sinai Peninsula, killing at least one soldier. Masked assailants launched a coordinated attack with rockets, rocket-propelled grenades and anti-aircraft guns on the airport in the northern Sinai city of el-Arish, where military aircraft are located, as well as a security forces camp in Rafah on the border with Gaza and five other military and police posts, sparking nearly four hours of clashes.
One of military's top commanders, Gen. Ahmed Wasfi arrived at el-Arish on Friday to lead operations there as the army declared a "war on terrorism" in Sinai. A crowd of Morsi supporters tried to storm the governor's office in the city but were dispersed by security forces.
The night of Morsi's ouster, jihadi groups held a rally in el-Arish attended by hundreds, vowing to fight. "War council, war council," a speaker shouted, according to online video of the rally. "No peacefulness after today."
Islamic militants hold a powerful sway in the lawless and chaotic northern Sinai. They are heavily armed with weapons smuggled from Libya and have links with militants in the neighboring Gaza Strip, run by Hamas. After the attack, Egypt indefinitely closed its border crossing into Gaza, sending 200 Palestinians back into the territory, said Gen. Sami Metwali, director of Rafah passage.
At the Rabia al-Adawiya rally earlier in the day, the crowd filled much of a broad boulevard, vowing to remain in place until Morsi is restored. The protesters railed against what they called the return of the regime of autocrat Hosni Mubarak, ousted in early 2011.
"The old regime has come back ... worse than before," said Ismail Abdel-Mohsen, an 18-year old student among the crowds outside the Rabia al-Adawiya Mosque. He dismissed the new interim head of state sworn in a day earlier, senior judge Adly Mansour, as "the military puppet."
"After sunset, President Morsi will be back in the palace," they chanted. "The people want God's law. Islamic, Islamic, whether the army likes it or not."
Many held copies of the Quran in the air, and much of the crowd had the long beards of ultraconservative men or encompassing black robes and veils worn by women, leaving only the eyes visible. One protester shouted that the sheik of Al-Azhar — Egypt's top Muslim cleric who backed the military's move — was "an agent of the Christians" — reflecting a sentiment that the Christian minority was behind Morsi's ouster.
In southern Egypt, Islamists attacked the main church in the city of Qena on Friday. In the town of Dabaiya near the city of Luxor, a mob torched houses of Christians, sending dozens of Christians seeking shelter in a police station. Clashes broke out Friday in at least two cities in the Nile Delta between pro- and anti-Morsi demonstrators.
The first steps for creating a post-Morsi government were taken Thursday, when Mansour, the 67-year-old chief justice of the Supreme Constitutional Court, was sworn in by fellow judges as interim president. A Cabinet of technocrats is to be formed to run the country for an interim period until new elections can be held — though officials have not said how long that will be. In the meantime, the Islamist-written constitution has been suspended.
On Friday, Mansour dissolved the country's interim parliament — the upper house of the legislature, which was overwhelmingly dominated by Islamists and Morsi allies. The Shura Council, which normally does not legislate, held legislative powers under Morsi's presidency because the lower house had been dissolved.
Mansour also named the head of General Intelligence, Rafaat Shehata, as his security adviser.