Security forces stormed the Cairo protest camps of ousted Islamist president Mohamed Morsi's supporters in a long-anticipated assault Wednesday that officials said led to almost 280 deaths across Egypt.

In response to the violence, the army-backed interim government imposed a month-long nationwide state of emergency and curfews in Cairo and 13 other provinces.

Authorities said 278 people were killed, including 43 policemen, with many of the deaths in Cairo but with the violence spreading from the capital and claiming lives across the country.

The state of emergency went into effect at 4:00 pm (1400 GMT), with what will be a daily 11-hour curfew beginning at 7:00 pm.

Gory photographs and video images of the Cairo bloodbath dominated social media networks, as world powers called for restraint and condemned the show of force by security forces.

At least four churches were attacked, with Christian activists accusing Morsi loyalists of waging "a war of retaliation against Copts in Egypt".

Hours after tear gas canisters first rained down on tents of protesters in the sprawling Rabaa al-Adawiya camp in eastern Cairo, an AFP correspondent counted at least 124 bodies in makeshift morgues there.

In a field hospital, its floors slippery with blood, doctors struggled to cope with the casualties, leaving the hopeless cases, even if still alive.

The health ministry said 235 civilians were killed in the Cairo crackdown and in subsequent clashes across Egypt.

The interior ministry added that 43 security personnel had lost their lives.

Among those killed in Cairo was 17-year-old Asmaa al-Beltagui, daughter of wanted Muslim Brotherhood leader Mohammed al-Beltagui, a spokesman for Morsi's movement said.

And Britain's Sky News said a veteran cameraman, Briton Mick Deane, was shot and killed while covering the violence.

Security officials had originally spoken of gradually dispersing the sit-ins over several days but the dramatic descent on the squares shortly after dawn came as a surprise to many.

The violence prompted vice president and Nobel laureate Mohamed ElBaradei to resign, saying his conscience was troubled over the loss of life, "particularly as I believe it could have been avoided."

"It has become too difficult to continue bearing responsibility for decisions I do not agree with and whose consequences I fear," he said.

By Wednesday evening, a security official said Rabaa al-Adawiya was "totally under control. There are no more clashes."

The authorities later said calm had been restored across the country.

Earlier, hundreds of Morsi supporters were given safe passage out of the camp, some flashing victory signs as they left through a security corridor.

A security official confirmed to AFP hundreds of people were taking advantage of the safe passage offer but said that some diehards had stayed behind to fight on.

Shortly after dawn, witnesses and an AFP correspondent said security forces fired tear gas before surging into Rabaa al-Adawiya, sparking pandemonium among the thousands of protesters who had set up the camp soon after Morsi was ousted in a July 3 military coup.

Men in gas masks rushed to grab each canister and dunk them in containers of water, as the main stage near the mosque of the camp blared Islamic anthems and protesters chanted "Allahu Akbar" (God is greatest.)

In the smaller of the protest camps, at Al-Nahda square in central Cairo, police said they took control of the square after two hours.

Television footage showed flattened tents, as women and children flanked by police and army troops were led out of the square.

Dozens rounded up in the dispersal were shown sitting on the ground, handcuffed and surrounded by security forces.

-- Use of force widely condemned --

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Europe's leading powers, along with Iran, Qatar and Turkey, strongly denounced the use of force by the interim government.

The White House said Washington, which provides Egypt with massive military aid, "strongly condemns" the violence against the protesters and opposes the imposition of a state of emergency.

Interim prime minister Hazem al-Beblawi praised the police for their "self-restraint" and said the government remained committed to an army-drafted roadmap that calls for elections in 2014.

Morsi's Muslim Brotherhood urged Egyptians to take to the streets in their thousands to denounce the "massacre."

"This is not an attempt to disperse, but a bloody attempt to crush all voices of opposition to the military coup," Brotherhood spokesman Gehad al-Haddad said on Twitter.

But Interior Minister Mohamed Ibrahim said no more protests would be tolerated.

The anger against the Islamist movement was evident as residents of several neighbourhoods clashed with Morsi loyalists.

Clashes also erupted between security forces and Morsi supporters in the northern provinces of Alexandria and Beheira, the canal provinces of Suez and Ismailiya and the central provinces of Assiut and Menya.

In Alexandria, hundreds of angry Morsi supporters marched through the streets armed with wooden clubs chanting "Morsi is my president."

An AFP reporter said they set fire to tyres and tore down pictures of army chief Abdel Fattah al-Sisi, who led the coup.

It was a dramatic turn of events for the Muslim Brotherhood, who just over a year ago celebrated Morsi's victory as Egypt's first elected president.

But Morsi's turbulent year in power, marred by political turmoil, deadly clashes and a crippling economic crisis, turned many against the Islamist movement.

On June 30, millions took to the streets to call on the army to remove Morsi.