At least 50 people, including police officers, have reportedly been killed after parts of Egypt descended into further violence.
Tens of thousands of Muslim Brotherhood supporters clashed with armed vigilantes in the fiercest street battles to engulf the capital since the country's Arab Spring uprising.
Residents battled with protesters taking part in what the Brotherhood called the "day of rage", ignited by anger at security forces for clearing two pro-Mohamed Morsi sit-in demonstrations on Wednesday.
State media said a police officer died following an armed attack on a checkpoint in the capital Cairo, where the army has been deployed to guard "important and vital facilities" - and is authorised to use live ammunition.
Witnesses reported seeing the bodies of at least 20 people in rows in a mosque in the centre of the city.
A man points a firearm during protests in Egypt
AFP reported that around 12 corpses had been taken to another mosque.
Automatic gunfire echoed across Cairo and black smoke billowed from the capital's huge Ramses Square, with a military helicopter hovering overhead looking down on the chaos.
Security officials said at least 12 people died in the square after protesters clashed with residents in the area.
Footage being broadcast on state TV in the country purported to show armed protesters shooting at security forces in Cairo, where tens of thousands have taken to the streets.
Reuters reported a security official who said 24 policemen have been killed in the last 24 hours.
Sky's Foreign Affairs Editor Tim Marshall said: "State TV's narrative is that this is terrorism, that the (pro-Morsi) Muslim Brotherhood is behind it and that the armed forces are on the right side and are protecting people against what they call terrorism.
"There are few other sources of visual information so far."
Soldiers stand guard in vehicles outside Egypt's state TV building
Tear gas could be seen during flashpoints in parts of the city, with protesters apparently hurling bottles and rocks at security forces.
AFP said four protesters were killed during battles with security forces in the Suez Canal city of Ismailiya.
Reuters said the casualties included four people in central Cairo, five in Fayoum and quoted medical sources as saying eight were killed in Damietta - around 120 miles from the capital.
It comes after hundreds were killed on Wednesday in a crackdown by security forces on supporters of ousted president Mohamed Morsi.
Egyptian armoured personnel carriers also gathered in Cairo's Tahrir Square
Thousands of pro-Morsi supporters have been marching from north-east Cairo towards the city's centre, chanting "down with military rule".
Barbed wire, armoured personnel carriers and armed soldiers were seen blocking an entrance to Tahrir Square, next to the Egyptian Museum, as well as side streets in downtown Cairo.
Islamist group the Muslim Brotherhood, who back the deposed leader, had earlier issued calls for a nationwide "day of rage" by millions of supporters following Wednesday's deadly clashes.
"Despite the pain and sorrow over the loss of our martyrs, the latest coup makers' crime has increased our determination to end them," it said in a statement.
In a counter move, the National Salvation Front - a loose liberal and leftist coalition - called on Egyptians to take to the streets today against what it said was "obvious terrorism actions" conducted by the Brotherhood.
The Brotherhood accuses the military of staging a coup last month when it ousted Mr Morsi, Egypt's first freely elected president since Hosni Mubarak was toppled.
Muslim Brotherhood supporters say they will not back down
Liberal and leftist activists who backed the military saw the move as a positive response to public demands.
Catherine Ashton's European External Action Service said top officials of all 28 European Union members would meet on Monday to review the crisis in Egypt and discuss possible EU action.
She has urged Europe to agree on "appropriate measures".
Prime Minister David Cameron and French President Francois Hollande are to hold talks about the ongoing unrest during a phone call this afternoon, while Germany said it condemned the violence in Egypt "in the strongest terms".
However, Saudi Arabia's King Abdullah said his country stood with Egypt in its battle against "terrorism".
Today's protest calls raised fears of renewed violence after nationwide clashes following Wednesday's operation to clear two protest camps in Cairo supporting Mr Morsi left more than 600 dead, including Sky News cameraman Mick Deane.
There were fresh attacks on security forces during a tense day on Thursday, with at least seven soldiers and a policeman killed in the Sinai peninsula and another police officer killed in the central city of Assiut.
Officials say more than 600 people died in nationwide clashes on Wednesday
With the country under a state of emergency and many provinces hit by night-time curfews, the interior ministry ordered police to use live ammunition if government buildings came under attack.
International criticism of the bloodshed has continued to pour in and the United Nations Security Council held an emergency meeting last night on the crisis at the request of France, Britain and Australia.
Afterwards, the Argentinian president of the council urged all sides to exercise "maximum restraint".
Ambassador Maria Cristina Perceval, whose country currently presides over the 15-country body, said member states called for an end to the violence and spoke of the need to advance "national reconciliation".
A military helicopter flies over Cairo
Signalling his displeasure at the worst bloodshed in Egypt for generations, US President Barack Obama said normal co-operation between Washington and Cairo could not continue and announced the cancellation of military exercises with Egypt next month.
"We deplore violence against civilians. We support universal rights essential to human dignity, including the right to peaceful protest," he said.
His remarks sparked a defiant response from the Egyptian presidency which said that "statements not based on facts may encourage violent armed groups".
Despite the condemnation, Egypt's interim prime minister Hazem al Beblawi praised the police for their "self-restraint" and said the government remained committed to an army-drafted roadmap calling for elections in 2014.
He justified the use of force saying supporters of Mr Morsi had been sowing chaos, "terrorising citizens, attacking public and private property".
Egypt's ambassador to Britain has said the force used in deadly raids on Cairo protest camps was "not at all excessive".
Speaking at the Egyptian embassy in London, Ashraf El Kholy said the camps set up by Morsi loyalists were "not as peaceful and innocent" as protesters claimed.
"I think with these number of deaths and this amount of violence, (the protesters) got what they wanted," he said.
While several countries, including the US and France, have advised their citizens not to travel to anywhere in Egypt, UK holiday companies are still operating in parts of the country.
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