* Muslim Brotherhood calls for protest marches
* Overnight curfew saw city streets largely calm
* Rage among Islamists, foreign governments deplore bloodshed
* Nearly 250 shrouded bodies lined up in Cairo mosque
By Tom Perry and Michael Georgy
CAIRO, Aug 15 (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 polarised and in turmoil.
A Reuters witness counted 228 bodies, most of them wrapped in white shrouds, arranged in rows on the floor of the Imam mosque in northeast Cairo, close to the worst of Wednesday's violence between police and demonstrators.
Islamist supporters of former President Mohamed Mursi, ousted by the army on July 3, clashed with police and troops who used bulldozers, teargas and live ammunition to clear two Cairo sit-ins that had become a hub of resistance to the military.
The clashes spread quickly, and a health ministry official said on Thursday that 525 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.
The capital and other areas hit by shocking violence 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 Mursi's supporters, most of them from his Muslim Brotherhood movement, is unclear.
The next potential flashpoint comes later on Thursday, after Mursi's Brotherhood movement called for marches in the capital to protest the deaths. In Alexandria, Egypt's second biggest city, protesters were already on the move by early afternoon.
A military source said that while sit-ins like the main one outside the Rabaa al-Adawiya mosque in Cairo would no longer be tolerated, marches may be in spite of the state of emergency.
Funerals of those killed will also be held throughout the day, and on Friday, the main Muslim day of prayer, anger could spill on to the streets.
"I think tomorrow will be a big day for protests throughout Egypt, with the potential for violence very high," said Yasser El-Shimy, Egypt analyst with the International Crisis Group.
WESTERN ANGER, UAE APPROVAL
The latest crackdown was the third mass killing of Islamist demonstrators since Mursi was deposed six weeks ago, but the scale of the bloodshed took many by surprise and signalled that the military was determined to tighten its grip on the country.
The decision to forcibly clear sit-ins defied Western appeals for a negotiated settlement to the crisis, amid concerns that the country which signed a peace treaty with Israel and straddles the strategic Suez Canal could spiral out of control.
French President Francois Hollande summoned the Egyptian ambassador to demand an immediate halt to the crackdown.
"The head of state asserted that everything must be done to avoid civil war," the Elysee Palace said in a statement on Thursday. "Freeing prisoners, in respect of legal proceedings, could be a first step towards the resumption of talks."
In Ankara, Turkish Prime Minister Tayyip Erdogan called 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".
At the site of one of the Cairo sit-ins, 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 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.
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 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 neighbouring building after a mob surrounded and hurled bricks at their place of worship, state news agency MENA said.
Authorities on Thursday referred 84 people from the city of Suez, including Muslim Brotherhood members, to military prosecutors on charges of murder and burning churches, the state news agency reported.
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 800, mostly Islamist supporters of the deposed president.
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.
After Mursi's ouster, Gulf Arab states pledged $12 billion in aid to Egypt, bolstering its coffers after reserves of foreign currency and food stocks had run dangerously low.
But in the first sign of adverse economic impact from the bloodshed, home appliances maker Electrolux said it halted all output in Egypt, where it has around 7,000 employees, and would review its decision on Saturday.