Egypt's interim president promised to hold parliamentary elections within a six-month transitional period on Monday after more than 50 supporters of ousted President Mohamed Morsi were killed and 435 others injured in clashes with the army.

Military-backed interim leader Adly Mansour late Monday decreed a six-month transition and said parliamentary elections would be held within that period.

The violence in the restive country erupted outside the headquarters of the elite Republican Guard, where Morsi was first detained last week.

The Muslim Brotherhood said those killed were mostly supporters of Morsi and accused Egyptian security forces of firing on protesters during dawn prayers.

Morsi's Muslim Brotherhood, which has led demonstrations against Wednesday's military overthrow of the Islamist president, has called for an "uprising." The party claimed troops and police "massacred" its supporters during dawn prayers in Cairo Monday.

The official MENA news agency reported that at least 51 protesters were killed in the violence. MENA also said that two police officers were killed and six others injured. The army said one officer was killed and 40 soldiers were wounded.

The army said that the violence was provoked by protesters who opened fire on the Republican Guard headquarters.

"We dealt with the protesters in the vicinity of the Republican Guard House wisely, while they used live ammunition, Molotov cocktails, stones and others," armed forces spokesman Colonel Ahmed Mohamed Ali said.

The ultraconservative Islamist Al-Nur party, which won almost a quarter of the votes in 2011-2012 parliamentary elections and had backed the ouster of Morsi, said it was quitting talks with Egypt's new rulers on forming a government because of what it called a "massacre" at the Republican Guard headquarters.

The Egyptian presidential office ordered a judicial inquiry to investigate the violence.

UN Secretary-General Ban Ki-moon on Monday urged all sides to act with maximum restraint.

"The secretary-general condemns these killings and calls for them to be thoroughly investigated by independent and competent national bodies, and those responsible need to be brought to justice," Ban's spokesperson said.

France on Monday urged all parties to "exercise restraint" to avoid further violence.

"France condemns the violence that broke out these past days, wherever it comes from," said Philippe Lalliot, a Foreign Ministry spokesman. "All light must be shed on the violence that left more than 50 dead this morning."

The White House said Monday that the United States is not calling the Egyptian military's ouster of the country's first elected president a coup but needs time to review what has taken place.

Morsi's lone year in office was marked by charges that he failed the 2011 revolution that deposed President Hosni Mubarak by concentrating power in Islamist hands and permitting the economy to suffer.

The military, which ousted Morsi after massive protests from June 30, has come under growing international pressure to quickly establish a civilian administration to oversee a rapid return to elected government.