The latest violence in Egypt, in which nearly 120 people have died, has sparked severe condemnation from across the world.
The US has urged Egypt to "step back from the brink" after the security forces opened fire on Muslim Brotherhood supporters.
"In this extremely volatile environment, Egyptian authorities have a moral and legal obligation to respect the right of peaceful assembly and freedom of expression. Both are essential components of the inclusive democratic process they have publicly embraced," said US Secretary of State, John Kerry.
The clashes led to massive rival rallies, threatening to plunge the Arab world's most populous nation into a deeper crisis.
Kerry also urged the Egyptian army and the interim administration to heed demands of the peaceful protesters.
"At this critical juncture, it is essential that the security forces and the interim government respect the right of peaceful protest, including the ongoing sit-in demonstrations."
UN Secretary General Ban Ki-moon has also expressed concern over the situation. The UN chief's spokesperson said: "The secretary-general once again calls on the interim authorities to assume full responsibility for the peaceful management of the demonstrations and ensure the protection of all Egyptians."
Apart from the heavy death toll, more than 1,000 people have been injured during the protests.
The "excessive use of force" by the military has also been condemned by the country's vice-president and Nobel laureate, Mohamed ElBaradei.
The liberal leader wrote on his Twitter account: "I strongly condemn the excessive use of force and the deaths, and I am working hard and in every direction to end the confrontation in a peaceful way, God protect Egypt and have mercy on the victims."
The National Salvation Front (NSF) in Egypt, the main opposition group when the Brotherhood ruled, has also condemned the killings but put the blame on supporters of deposed president Mohamed Morsi.
"Based on reports of the committee, all those responsible must be held accountable, including the minister of interior, if it is proven that the security forces were involved in excessive use of force against protesters," the NSF said in a statement.