EGYPTIAN security forces crushed a protest camp of thousands of supporters of the deposed president, shooting dead scores of people in the bloodiest day in decades in the Arab world's biggest country.

The Health Ministry said at least 278 people were killed, both in Cairo and in clashes that broke out elsewhere in the country. Deposed President Mohamed Mursi's Muslim Brotherhood said the death toll was far higher in what it described as a "massacre".

While dead bodies wrapped in carpets were carried to a makeshift morgue near the Rabaa al Adawiya mosque in the capital, the army-backed rulers declared a one-month state of emergency, restoring to the military the unfettered power it wielded for decades before a pro-democracy uprising in 2011.

Thousands of Mursi's supporters had been camped at two major sites in Cairo since before he was toppled on July 3 and had vowed not leave the streets until he was returned to power.

Violence spread beyond 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.

With yesterday's assault on the camps, the authorities have ended the six-week stand-off with a show of state force that defied international pleas for restraint.

The bloodshed also effectively ends the open political role of the Brotherhood, which survived for 85 years as an underground movement before emerging from the shadows after the 2011 uprising to win every election held since.

In one a rare sign of unease from among the Brotherhood's opponents, Mohamed ElBaradei, a former UN diplomat, quit his post of vice president in the army-backed government, saying the conflict could have been resolved by peaceful means.

After the assault on the camp began, desperate residents recited Koranic verse and screamed "God help us, God help us" while helicopters hovered overhead and armoured bulldozers ploughed over their makeshift defences.

At one stage, masked police in dark uniforms poured out of police vans with sticks and tear gas bombs before tearing down tents and setting them ablaze.

After shooting with live ­ammunition began, wounded and dead lay on the streets near pools of blood. An area of the camp that had been a playground and art exhibit for the children of protesters was turned into a war-zone field hospital.

The government insists people in the camp were armed. Several television stations, all controlled by the state or its sympathisers, ran footage of what appeared to be pro-Mursi protesters firing rifles at soldiers from behind sandbag barricades.

The violence forces tough ­decisions for Egypt's Western allies, especially Washington, which funds Egypt's military with £800 million a year and has refused to label its overthrow of Mr Mursi a "coup".

White House spokesman Josh Earnest said: "The United States strongly condemns the use of violence against protesters in Egypt. We have repeatedly called on the Egyptian military and security forces to show restraint. The world is watching what is happening in Cairo."

Outside of Cairo, state media said Mursi supporters besieged and set fire to government buildings and attacked several churches. Those reports could not be independently confirmed.

Turkey urged the UN Security Council and Arab League to act quickly to stop a "massacre" in Egypt. Iran warned of the risk of civil war. The European Union and several of its member countries deplored the killings.

Hours after the start of yesterday's operation, protesters were still blocking roads, chanting and waving flags as the military tried to prevent them from regrouping.