More than 20 people have died in fresh violence in Egypt, as protesters loyal to the ousted President Mohammed Morsi clash again with security forces.
The BBC's Jeremy Bowen has seen 11 bodies and many people badly injured near Cairo's Ramses Square.
Two days ago the protesters' camps were broken up, leaving at least 638 dead and causing international condemnation.
But the protests are not just in Cairo. At least 12 people have been reported dead elsewhere in Egypt on Friday.
Eight were reportedly killed in Damietta, and four in Ismailia.
In the wake of Wednesday's violence, the interior ministry says police have been authorised to use live ammunition "within a legal framework".
'March of Anger'
Hundreds of people gathered at a mosque in Ramses Square on Friday, after the Muslim Brotherhood, of which Mr Morsi is a member, appealed to its supporters to join a "march of anger".
The demonstrations are taking place under the slogan "the people want to topple the coup" - referring to the military's removal of Mr Morsi on 3 July.
The protests quickly became violent - the BBC's Jeremy Bowen says the trigger was when a police station came under fire.
He said he could hear tear gas being used and then live ammunition - long bursts of automatic fire.
From where he was standing - near a mosque not far from the square - he said he could see many casualties being brought in for help.
Other correspondents say gunfire is also being heard in Garden City on the banks of the Nile - and smoke is pouring from the front of the Four Seasons Hotel, also on the river.
"Egypt is in a deepening, really serious crisis - I can't see how they can get out of it without a lot more violence and death," our correspondent says.
Security in the capital is tight, with many armoured personnel carriers on the streets.
Entrances to Tahrir Square, the focus of demonstrations that led to the toppling of President Hosni Mubarak in 2011, were also reported to have been blocked off by the army.
State TV said the military was deploying to protect "important and vital facilities".
Members of groups opposed to Mr Morsi - the National Salvation Front and Tamarod - are reported to have called for counter-demonstrations in response to the Muslim Brotherhood protests.
There have also been calls for people to protect their neighbourhoods and churches throughout the country.
Egypt's Coptic Christian community has been targeted by some Islamists who accuse the Church of backing the army's overthrow of Mr Morsi last month.
'Maximum restraint call'
Meanwhile, Egypt's interim leaders have criticised remarks by President Barack Obama on Thursday.
Mr Obama condemned the Egyptian government's actions in ordering security forces to break up the protest camps, and cancelled joint military exercises.
He said co-operation could not continue while civilians were being killed. However, he stopped short of cutting the $1.3bn (£830m) in aid that the US gives to Egypt.
The Egyptian presidency said in a statement that Mr Obama's words were "not based on fact" and would "embolden armed groups".
Wednesday's bloodshed has drawn widespread international condemnation.
French President Francois Hollande and German Chancellor Angela Merkel have called for urgent European consultations on the escalating crisis.
EU diplomats will meet in Brussels on Monday to discuss their response. Some have already suggested that European aid to Egypt should be frozen.
EU foreign policy envoy Catherine Ashton said responsibility for the crisis "weighs heavily on the interim government, as well as on the wider political leadership in the country".
Turkey has described Wednesday's events as a "massacre" and recalled its ambassador to Cairo "to discuss the latest developments". In retaliation, Cairo has cancelled naval exercises with Turkey.
US Republican Senator John McCain told BBC Newsnight that the ousting of President Morsi had been a "coup" and President Obama should have cut off aid to Egypt as a result.
The US has so far been careful not to use the word coup as under US law this would mean stopping aid.
But some other nations support the interim government's actions.
Saudi Arabia's King Abdullah issued a statement saying: "The Kingdom of Saudi Arabia, its people and government stood and stands by today with its brothers in Egypt against terrorism."
Wednesday's violence began when armoured bulldozers moved into the two Cairo protest camps.
The smaller of the two camps, at Nahda Square, was cleared quickly but clashes raged for several hours in and around the main encampment near the Rabaa al-Adawiya mosque. The mosque was badly damaged by fire.
Egyptian interim Prime Minister Hazem Beblawi said the authorities had to restore security.
Mr Morsi, Egypt's first democratically elected president, is in custody accused of murder over a 2011 jailbreak. His period of detention was extended by 30 days on Thursday, state media said.
Correction 30 August 2013: The caption on the sixth image in the picture gallery has been changed as it is unclear whether the gunman was a supporter of the Muslim Brotherhood as originally stated.