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CAIRO - Clashes between security forces and protesters killed at least another80 people Friday, including 10 police officers, as thousands ofsupporters of the Muslim Brotherhood took to streets in defiance of amilitary-imposed state of emergency.

The day after marches in Cairo devolved into the fiercest street battles that the capital has seen in more than two years, supporters of Egypt's ousted Islamist President Mohamed Morsi were defying a state of emergency with new protests.

As the sun rose Saturday morning over Egypt's capital, security forces were in standoff with Morsi backers. Hundreds of people barricaded themselves overnight inside the al-Fatah mosque, shoving furniture against the doors to stop police from breaking their way in.

Police were negotiating with those inside, promising them safe passage if they leave. Small groups were seen emerging from the mosque by late morning, but more are believed to be still holed up inside.

The mosque at Ramses Square, scene of some of the heaviest clashes Friday, had been used as a field hospital and morgue earlier in the day.

About 700 people have now been killed in clashes this week.

An eerie calm swept over the city Friday after a curfew set in at 7 p.m., local time. It's an hour that typically sees the city bustling with movement. But after another day of violence, the streets were so calm that one could hear the rustling leaves of trees.

Near the site of one set of clashes that erupted earlier in the day, plainclothes and uniformed police patrolled the streets along with other packs of security forces positioned across the capital. They stopped and questioned every passing vehicle, which were few and far between.

"It's not safe here," a plainclothes policeman warned, not giving his name because he was not authorized to speak to the media.

As the dust from Friday's violence cleared, the Muslim Brotherhood called for a week of more protests. They will march from the same locations across Egypt until they break the coup that ousted Brotherhood leader and Egyptian president Mohamed Morsi.

Earlier Friday, protesters poured out of mosques after traditional mid-day prayers in response to the Muslim Brotherhood's call for a "Friday of Anger" against Morsi's July ouster and the deadly violence during a police operation to evict Morsi supporters from protest camps.

Unlike in past clashes between protesters and police, Friday's turmoil took an even darker turn when residents and possibly police in civilian clothing engaged in the violence. Uniformed police were nowhere to be seen as residents fired at one another on a bridge that crosses over Zamalek in Cairo, an upscale island neighborhood where many foreigners and ambassadors reside.

"We don't have weapons. We don't have Kalashnikovs," said Jamal Salam, wearing a robe and with a long grey beard as a helicopter buzzed overhead in the hot afternoon. "And they are shooting us."

Protesters vowed to keep up their demonstrations against Gen. Abdel Fattah Al-Sisi who led the July 3 overthrow of former president Morsi.

Armored military vehicles sealed off main squares in the Egyptian capital and troops with machine guns stood at the ready on key junctions. Mohammed Attiya, a pro-Morsi protester speaking via phone from Tanta, a city north of Cairo, said security forces were firing tear gas at demonstrators there as well.

"We can't breathe. We can't see," he said.

At least 638 were killed nationwide Wednesday including 43 police, the Health Ministry said. Most died in violence at two main protest sites positioned on opposite sides of the capital. With bulldozers, tear gas and live ammunition, security forces tore through the sit-ins, where protesters gathered for six weeks denouncing the military coup and demanding Morsi's reinstatement.

The government defended its position, saying it gave protesters a chance to leave and was "keen to adopt a gradual plan to avoid bloodshed and falling of victims."

Morsi was ousted after millions of Egyptians took to the streets to protest what they called dictatorial policies that overrode their recent democracy following the 2011 ouster of longtime dictator Hosni Mubarak by the military. An interim government was appointed by the military to create a plan for new elections.

But the Brotherhood refuses to participate, demanding the return of Morsi, who is being held under house arrest.

"All ways to peacefully end the two sit-ins were in vain," a State Information Service statement said, adding that suggested initiatives to do so were welcomed but rejected by leaders of the Muslim Brotherhood.

Friday, Khaled Dawoud resigned from his position as spokesperson of the National Salvation Front, a coalition of leftist and liberal groups formerly led by Mohamed ElBaradei, who was appointed interim vice president for foreign affairs in July.

Dawoud cited the National Salvation Front's refusal to condemn Wednesday's "massacre" as the reason for his resignation, as well as the "vicious attacks" against ElBaradei after the vice president resigned from his government post two days ago.

"I think the bloody breakup of the Rabaa and Nahda sit-ins should have been condemned and there should have been a clear indication that this is not what we're aiming for," he said.

"I don't want to be involved in any party or front that finds the acts of police justified," he said.

On Thursday, demonstrators attacked and torched a government building in Giza not far from the second protest site, which was completely cleared and the roads open to traffic.

Pro-Morsi Egyptians attacked churches of Coptic Christians, apparently blaming the Christians for the ouster or Morsi. Christians make up about 10% of Egypt's 83 million people. The protesters also attacked numerous police stations after the sit-ins were cleared out, the Interior ministry had said. Forty-three police officers were killed, it said.

The State Department urged U.S. citizens living in Egypt to depart and warned Americans to defer travel to the country "because of the continuing political and social unrest." Last month, the State Department ordered non-emergency government personnel and their families to leave Egypt.

President Obama's cancelled joint military exercise with Egypt and calling on all sides to solve their differences without violence.

"While we want to sustain our relationship with Egypt, our traditional cooperation cannot continue as usual when civilians are being killed in the streets and rights are being rolled back," Obama said Thursday, speaking from his weeklong vacation in Martha's Vineyard.

A Working Group on Egypt - comprised of eleven policy advisers, co-chairs, executives, directors and senior fellows at Freedom House, the Council on Foreign Relations and nine other institutes - called on the Obama administration to take further steps, including immediate suspension of military aid that totals $1.3 billion annually.

Washington has refused to call the military takeover in Egypt a coup, which would require the U.S. to halt funding.

"The killing of hundreds of protesters carried out by the Egyptian military government was unnecessary, unjustified, and in contravention of international human rights standards," the Working Group on Egypt said in a statement. "These events demand a shift in U.S. policy that is urgent and long overdue."

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