Egypt's Muslim Brotherhood called Thursday for fresh marches to protest the killing of at least 525 people in a police crackdown on its supporters that deepened a political crisis sparked by the military's ouster of Islamist president Mohammed Morsi, dpa reported.
The Interior Ministry said it had authorized police to shoot at demonstrators who attack government buildings.
The announcement came after Brotherhood supporters, angered by the bloody assault on two of its protest camps a day earlier, stormed and set fire to government offices in Giza, Cairo's twin city. Other protesters attacked police stations and Coptic Christian churches across Egypt.
The Brotherhood, from which Morsi hails, had earlier vowed to bring down the "military coup" through peaceful means, undeterred by a dusk-to-dawn curfew imposed by the army-appointed government in Cairo and 13 other governorates.
Police on Wednesday used tear gas, bulldozers and live bullets to clear protesters from the sprawling encampments where Morsi supporters had been demanding the deposed leader's reinstatement for six weeks.
At least 289 people were killed at the sit-ins in the north-east and south of Cairo, triggering deadly violence across the Arab world's most populous nation of 85 million people.
More than 3,700 have been injured, the Health Ministry said.
A dpa witness on Thursday counted at least 230 bodies in white burial shrouds in a mosque near the area of Rabaa Al Adawiya, where Morsi's supporters had set up their main protest camp.
Ice blocks had been placed on some of the corpses to hold off decay.
Grieving women sat by others, awaiting the arrival of medical examiners to release burial certificates.
Men sprayed air freshener around to mask the smell of death and doused the floor with disinfectant. Coffins, electric fans and new consignments of ice were constantly arriving.
Outside, people searched for their relatives' names in the lists that covered 19 posters hanging on the mosque's boundary fence.
"This is a war against Islam," said Diya Ragab as he stood on the footpath holding burnt remains of a Koran copy he said he had rescued after the Rabaa al-Adawiya mosque was set on fire.
In volatile Sinai, six soldiers were killed and seven injured in an attack on a security checkpoint, said security sources.
The attack, believed to be mounted by Islamist insurgents, is the latest in the vast desert peninsula since Morsi's toppling on July 3.
Wednesday's violence, Egypt's deadliest since the revolution that forced Hosny Mubarak to resign in 2011, drew condemnation from leaders around the world, including US President Barack Obama.
He said that the US was cancelling joint military exercises planned for next month with Egypt.
Obama interrupted his holidays in Martha's Vineyard, Massachusetts, to deliver a statement that the US strongly condemns the actions of Egyptian security forces and deplores violence against civilians.
He said the US opposed the institution of martial law and the trampling of individual freedoms.
"While we want to sustain our relationship with Egypt, our traditional cooperation cannot continue as usual when civilians are being killed in the streets and rights are being rolled back," Obama said.
He pointed to the decades-long cooperation between the two countries, saying the US would review further steps in its relationship, but did not directly mention the status of 1.5 billion dollars in aid that primarily goes to the Egyptian military.
Last month, the US delayed the delivery of four F-16 fighter jets due to the unrest in the country.
Obama stressed that "America cannot determine the future of Egypt" and said Washington did not support any particular party or political figure.
The State Department also denounced the recent attacks on Coptic churches and public buildings, such as police stations.
"These attacks are just further aggravating an already fragile atmosphere," spokeswoman Jen Psaki said.
France, Britain Germany and Italy summoned Egyptian envoys to protest the crackdown on Islamist protesters.
European Union foreign ministers will hold a meeting next week to discuss the situation in Egypt.
Turkish Prime Minister Recep Tayyip Erdogan has demanded the United Nations Security Council convene to discuss the escalation.
The Brotherhood appeared determined to stick to its demand that Morsi, Egypt's first democratically elected president, be reinstated.
"We will always be non-violent and peaceful," Brotherhood spokesman Gehad El-Hadad wrote on Twitter. "We remain strong, defiant and resolute. We will push forward until we bring down this military coup."
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