Clashes between Islamist protesters and the army in Cairo have killed at least 42, according to a health official. Mohamed Sultan, the head of the Ambulance Authority, said 322 were injured.
The violence erupted at dawn on Monday and amplifies the conflict between the army, which ousted president Mohammed Morsi last week, and supporters of the Muslim Brotherhood group, who vowed to continue demonstrating until he is restored to power.
Fighting broke out when an armed group attempted to storm the Republican Guards club, where Morsi is thought to be held by the army.
The gunmen tried to penetrate the barbed wire surrounding the compound while snipers working in tandem fired from nearby rooftops, a military source told DPA, adding that more than 40 were injured in the incident.
The incident occurred while Morsi loyalists were staging a sit-in at the military building in eastern Cairo.
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The army arrested about 200 protesters. Morsi's Muslim Brotherhood group said 50 people were killed when army and police forces tried to disperse their encampment using lethal force.
"This heinous crime ... emphasises the extent of his brutality and determination to continue usurping power by walking on the skulls of unarmed civilians," the Brotherhood said, referring to army chief and Defence Minister Abdel-Fattah al-Sissi who led Morsi's overthrow.
Nationwide clashes between rival protesters have killed 72 people since June 28, the health ministry said according to Al-Ahram newspaper.
Meanwhile Egypt's prosecution ordered the closure of the Cairo headquarters of the Muslim Brotherhood's Freedom and Justice Party after police found weapons inside it on Monday, a security official said.
Police found "flammable liquid, knives and arms to be used against the June 30 protests," the official told AFP, in reference to demonstrations that saw millions take to the streets to demand the resignation of Islamist president Mohamed Morsi.
The ultraconservative Salafist party al-Nour said it was withdrawing from talks with Egypt's new rulers on forming a government after Monday's "massacre" at the military facility.
"We decided to withdraw immediately from all negotiations in response to the Republican Guard massacre," al-Nour spokesman Nader Bakkar said on social networking sites.
"We will not be silent," he added. "We wanted to stop bloodshed and now it is spilled in rivers."
Al Nour, which came second in Egypt's 2011 parliamentary elections, had previously backed the army's toppling of Morsi. Egyptian newspaper al-Ahram quoted sources as saying that Ziad Bahaa el-Din, an economist who was a founding member of the Egyptian Social Democratic Party, had been offered the premiership.
But there had been no "final nor official" decision on the post, the newspaper reported. Nobel laureate and former International Atomic Energy Agency director Mohamed El-Baradei was also offered the position of vice president.
