At least 12 people have
died in central Cairo, as supporters of ousted Egyptian President
Mohammed Morsi clash again with security forces.
Two days ago the protesters' camps were broken up, leaving at least 638 dead and triggering international condemnation.At least 12 people have also been killed in protests outside Cairo.
Eight people were reportedly killed in Damietta, and four in Ismailia.
In the wake of Wednesday's violence, police have been authorised to use live ammunition in self-defence.
Hundreds of people gathered at a mosque in Ramses Square after the Muslim Brotherhood, which backs Mr Morsi, appealed to its supporters to demonstrate after Friday prayers in 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.
But the protests quickly became violent - according to our correspondent 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 is standing - near a mosque not far from the square - he said he could see many casualties being brought in for help.
"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," he said.
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.
A protester in Cairo tells the BBC's Mishal Husain: "We are fighting for the dignity of Egypt"
The Egyptian Initiative for Personal Rights, an NGO, says 25 churches, along with private homes and businesses belonging to Copts and other Christian denominations, were attacked on Wednesday and Thursday.
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 been careful not to use the word coup as under US law this would mean stopping aid.
"I am confused that we would not enforce our own laws," he said.
"That sends a message of tolerance of brutality, of ineffective American leadership to the Muslim world, that the United States of America at least condones this kind of behaviour, which we cannot."
'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 in the early hours of Friday that Mr Obama's words were "not based on fact" and would "embolden armed groups".
It said Egypt was facing "terrorist acts".
Wednesday's bloodshed has drawn widespread international condemnation.
Late on Thursday, the UN Security Council met in emergency session at the request of France, Britain and Australia.
US Senator John McCain: "The law is very clear that if there is a coup aid is cut off"
Turkey, which described Wednesday's events as a "massacre", has recalled its ambassador to Cairo "to discuss the latest developments".
On Friday, the German government warned that further clashes "could plunge Egypt into a chaos of violence and counter-violence and further escalation".
It called for dialogue between the opposing parties, and also widened a travel alert for German citizens, advising them to stay away from the whole of Egypt.
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.
