A policeman and 16 protesters are reportedly dead as parts of Egypt descended into further violence.
State media said the officer died following an armed attack on a checkpoint in capital city Cairo, where the army has been deployed to guard "important and vital facilities" - and is authorised to use live ammunition.
Footage being broadcast on state TV in the country purported to show armed protesters shooting at security forces in Cairo.
News agency AFP said four protesters were killed during battles with security forces in the Suez Canal city of Ismailiya, while Reuters said four had died in central Cairo and quotes 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 Mohammed Morsi.
UN has appealed to all parties in Egypt to act with restraint Image: UN Photo
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.
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.
British Prime Minister David Cameron and French President Francois Hollande are due 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".
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.
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".
President of the UN Security Council, María Cristina Perceval, speaks to journalists following talks on Egypt Image: UN Photo
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".
