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Egypt's Muslim Brotherhood  returned to Egypt's streets Thursday after vowing to bring down the "military coup" with a "peaceful" struggle, despite the heavy loss of life when government forces broke up its protest camps Wednesday.

The death toll from the previous day's violence continued to climb Thursday. An Egyptian Health Ministry spokesman said that number had reached 638 by the evening. Mohammed Fathallah told The Associated Press that the number of injured in had also risen, and now stood at 3,994.

On Thursday afternoon, hundreds of Muslim Brotherhood supporters stormed a government building in Cairo and set a fire at its entrance, state TV and witnesses said. Hundreds marched in Egypt's second biggest city of Alexandria to protest the violence. "We will come back again for the sake of our martyrs," the marchers chanted. 

Meanwhile, seven Egyptian soldiers were shot dead by unknown gunmen near the city of El-Arish in the lawless north Sinai region, according to security and medical sources. In a separate incident, five soldiers were injured when gunmen opened fire on an army tent. 

Also on Thursday, Egypt's state news agency announced that judicial authorities have extended deposed President Mohammed Morsi's detention period for 30 days. Morsi, overthrown by the army on July 3, is being held at an undisclosed location on allegations of murder and spying.

Appeals for restraint

The crackdown on Wednesday defied Western appeals for restraint and a peaceful, negotiated settlement to Egypt's political crisis following the military's removal of Morsi last month, prompting international statements of dismay and condemnation.

"We will always be non-violent and peaceful. We remain strong, defiant and resolved," Brotherhood spokesman Gehad El-Haddad wrote on his Twitter feed. "We will push (forward) until we bring down this military coup," he added.

Security forces struggled to clamp a lid on Egypt after the worst nationwide bloodshed in decades, although a curfew largely held in Cairo overnight.

Islamists clashed with police and troops who used bulldozers, teargas and live fire on Wednesday to clear out two Cairo sit-ins that had become a hub of Muslim Brotherhood resistance to the military after it deposed Morsi. 

The clashes spread quickly, and a health ministry official said about 300 people were killed and more than 2,000 injured in fighting in Cairo, Alexandria and numerous towns and cities around the mostly Muslim nation of 84 million.

In Ankara, Turkish Prime Minister Tayyip Erdogan called on Thursday for the UN Security Council to convene quickly and act after what he described as a massacre in Egypt.

"Those who remain silent in the face of this massacre are as guilty as those who carried it out. The UN Security Council must convene quickly," he told a news conference.

At the site of one Cairo sit-in, garbage collectors cleared still-smouldering piles of burnt tents on Thursday. Soldiers dismantled the stage at the heart of the protest camp. A burnt out armoured vehicle stood abandoned in the street.

The Muslim Brotherhood said the true death toll was far higher, with a spokesman saying 2,000 people had been killed in a "massacre". It was impossible to verify the figures independently given the extent of the violence.

The military-installed government declared a month-long state of emergency and imposed the dusk-to-dawn curfew on Cairo and 10 other provinces, restoring to the army powers of arrest and indefinite detention it held for decades until the fall of autocrat Hosni Mubarak in a 2011 popular uprising.

The army insists it does not seek power and acted last month in response to mass demonstrations calling for Morsi's removal.

Vice President Mohamed ElBaradei, a Nobel Peace Prize winner who lent liberal political support to the ousting of Egypt's first freely elected president, resigned in dismay over the use of force, wishing to bring the six-week stand-off to an end through negotiation.

Other liberals and technocrats in the interim government did not follow suit. Interim Prime Minister Hazem el-Beblawi spoke in a televised address of a "difficult day for Egypt" but said the government had no choice but to order the crackdown to prevent anarchy spreading.

"We found that matters had reached a point that no self-respecting state could accept," he said.

Churches targeted

Islamists staged revenge attacks on Christian targets in several areas, torching churches, homes and business after Coptic Pope Tawadros gave his blessing to the military takeover that ousted Morsi, security sources and state media said.

Churches were attacked in the Nile Valley towns of Minya, Sohag and Assiut, where Christians escaped across the roof into a neighbouring building after a mob surrounded and hurled bricks at their place of worship, state news agency MENA said.

The United States, the European Union, the United Nations and fellow Muslim power Turkey condemned the violence and called for the lifting of the state of emergency and an inclusive political solution to Egypt's crisis.

Interior Minister Mohamed Ibrahim told a news conference 43 members of the police force were killed in the clashes.

He vowed to restore Mubarak-era security after announcing, in a statement last month that chilled human rights campaigners, the return of notorious political police departments that had been scrapped after the 2011 revolution.

U.S. Secretary of State John Kerry called the bloodshed in Egypt "deplorable" - a word U.S. diplomats rarely use - and urged all sides to seek a political solution.
A U.S. official told Reuters that Washington was considering cancelling a major joint military exercise with Egypt, due this year, after the latest violence, in what would be a direct snub to the Egyptian armed forces.

The "Bright Star" exercise has been a cornerstone of U.S.-Egyptian military relations and began in 1981 after the Camp David peace accords between Egypt and Israel. The United States has already halted delivery of four F-16 fighter jets in a signal of its displeasure.

Islamist militants with no direct link to the Brotherhood have staged almost daily attacks on security forces in the lawless Sinai Peninsula bordering Israel since Morsi's fall.

In the latest violence, gunmen shot dead two policemen outside their station in El Arish in northern Sinai on Wednesday evening, MENA reported.