U.N. rights chief Navi Pillay on Thursday demanded a wide-ranging probe into the bloody crackdown by Egyptian security forces on Muslim Brotherhood protesters.,

"There must be an independent, impartial, effective and credible investigation of the conduct of the security forces. Anyone found guilty of wrongdoing should be held to account," Pillay said in a statement.

Britain has summoned Egypt's ambassador to express its "deep concern" at the deadly violence in the country and urged the authorities to act with "the greatest restraint", the Foreign Office said on Thursday.

"Yesterday (Wednesday) we called in the Egyptian ambassador to express our deep concern at the escalating violence and unrest in Egypt," a spokesman said.

Egypt's military rulers Thursday faced international condemnation over the bloody crackdown on Muslim Brotherhood protesters, with France warning of the threat of "civil war" and Turkey demanding U.N. action.

At least 525 people were killed in Wednesday's assaults on two Cairo protest camps of supporters of ousted Islamist President Mohammed Morsi in the worst violence the country has seen in decades.

Pope Francis said on Thursday he was praying for the victims of the violence in Egypt that has left more than 500 people dead and appealed for "peace, dialogue and reconciliation".

"About the very sad news that has come unfortunately from Egypt: I wish to address my prayers to the victims, their families, the wounded and to those who are suffering," the pontiff said during a blessing at Castel Gandolfo, the summer papal residence near Rome.

Russia's foreign ministry on Thursday urged tourists to avoid trips to Egypt because of raging violence that has claimed more than 400 lives and spread to provinces outside the capital Cairo.

"Clashes and unrest that began in the capital are quickly spreading to other cities and regions of Egypt, including those favoured by tourists," ministry spokesman Alexander Lukashevich said in a statement.

Egyptian authorities have closed the Rafah border crossing with the Gaza Strip "indefinitely" for security reasons after a day of deadly violence nationwide, a security official told Agence France Presse on Thursday.

Hundreds of Palestinian travelers were left stranded on both sides of the crossing, the only gateway into the Hamas-ruled Palestinian territory that bypasses Israel, witnesses said.

The United Arab Emirates and Bahrain have separately voiced support for a deadly assault by Egypt's security forces on Islamist supporters of ousted president Mohammed Morsi, saying it was the state's duty to restore order.

The Emirati foreign ministry affirmed in a late Wednesday statement its "understanding of the sovereign measures taken by the Egyptian government after it had practiced maximum self-restraint during the preceding period."

Turkish Prime Minister Recep Tayyip Erdogan on Thursday called for an urgent U.N. Security Council meeting over Egypt's bloody crackdown on supporters of ousted Islamist president Mohammed Morsi, which has left hundreds dead.

"The Security Council of the United Nations should convene quickly to discuss the situation in Egypt," Erdogan told reporters in Ankara.

At least 578 people were killed in the violence that swept Egypt Wednesday, the health ministry said, with more than 300 of them losing their lives after police assaults on Cairo sit-ins.

The death toll included 43 policemen and 318 protesters killed in Cairo's Rabaa al-Adawiya and Nahda square protest camps, senior health ministry official Khaled al-Khatib told Agence France Presse Thursday.

Egypt's interim prime minister Hazem al-Beblawi praised police for their "self-restraint" on Wednesday and said the country remains committed to an army-drafted roadmap which calls for elections in 2014.

He was speaking after scores of people were killed in clashes with security forces who stormed two protest camps occupied for several weeks by supporters of ousted president Mohammed Morsi.