A torn poster of deposed Egyptian President Mohamed Mursi is seen as riot police clear the area of his supporters, at Rabaa Adawiya square, where they had been camping, in Cairo August 14, 2013. REUTERS/Mohamed Abd El Ghany CAIRO (Reuters) - Egypt's Muslim Brotherhood called on followers to march in protest in Cairo on Thursday, after at least 525 people were killed in a security crackdown on the Islamist movement that has left the most populous Arab nation polarized and in turmoil.

Cairo and other cities hit by shocking violence on Wednesday were largely calm overnight, after the army-installed government declared a month-long state of emergency and imposed a dusk-to-dawn curfew on the capital and 10 other provinces.

But whether the powerful military can keep a lid on the fury felt by millions of supporters of ousted President Mohamed Mursi, whom it deposed on July 3, is unclear.

The next potential flashpoint comes later on Thursday, after Mursi's Brotherhood movement called for marches in the capital to protest against the deaths.

His Islamist followers clashed with police and troops who used bulldozers, teargas and live ammunition on Wednesday to clear out two Cairo sit-ins that had become a hub of Brotherhood resistance to the military.

The clashes spread quickly, and a health ministry official said on Thursday that 421 people were killed and more than 3,500 injured in fighting in Cairo, Alexandria and numerous towns and cities around the mostly Muslim nation of 84 million.

It was the third mass killing of Islamist demonstrators since Mursi was deposed six weeks ago, but the scale of Wednesday's bloodshed took many by surprise and signaled that the military was determined to tighten its grip on the country.

The decision to forcibly clear the sit-ins defied Western appeals for restraint and a negotiated settlement to Egypt's political crisis, and there has been swift condemnation from many, though not all, countries.

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

"I am calling on Western countries. You remained silent in Gaza, you remained silent in Syria ... You are still silent on Egypt. So how come you talk about democracy, freedom, global values and human rights?" he told a news conference.

But the United Arab Emirates, one of several Gulf Arab states unsettled by Mursi's victory in a 2012 election, expressed support for the crackdown, saying the Egyptian government had "exercised maximum self-control".

TENTS STILL SMOULDER

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

The Muslim Brotherhood said the true death toll was far higher, with a spokesman saying 3,000 people had been killed in a "massacre". It was impossible to verify the figures independently given the extent of the violence.

The state of emergency and curfew restored to the army powers of arrest and indefinite detention it held for decades under autocrat Hosni Mubarak, ousted in a 2011 popular uprising.

The army insists it does not seek power and acted last month to remove Mursi in response to mass demonstrations calling for his resignation.

It has installed an interim government to implement plans for fresh elections in around six months, but efforts to restore democracy have been overshadowed by a political crisis that has deeply divided Egyptians between pro- and anti-Mursi camps.

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 at the use of force instead of a negotiated end to the six-week stand-off.

Other liberals and technocrats in the temporary 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

Violence rippled out from Cairo, with Mursi supporters and security forces clashing in the cities of Alexandria, Minya, Assiut, Fayoum and Suez and in Buhayra and Beni Suef provinces.

Islamists staged revenge attacks on Christian targets in several areas, setting fire to churches, homes and businesses after Coptic Pope Tawadros gave his blessing to the military takeover that ousted Mursi, 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 neighboring building after a mob surrounded and hurled bricks at their place of worship, state news agency MENA said.

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.

Wednesday's official death toll took the number of people killed in political violence since Mursi's fall to well over 700, mostly Islamist supporters of the deposed president.

The United States, the European Union and the United Nations condemned the violence and called for the lifting of the state of emergency and an inclusive political solution to the crisis.

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 warplanes in a signal of its displeasure.

(Additional reporting by Shadia Nasralla, Alexander Dziadosz, Michael Georgy and Tom Perry in Cairo, and Arshad Mohammed and Lesley Wroughton in Washington; Writing by Mike Collett-White; Editing by Michael Georgy)