Four people have been killed in Egypt as supporters of ousted President Mohammed Morsi clashed with opponents and security forces.
Medical officials said at least 40 others were hurt as gunfire and explosions rocked the centre of Cairo.
Troops used tear gas and live rounds to halt crowds heading to Tahrir Square.
The square has since been sealed off to prevent pro-Morsi supporters occupying the symbolic heart of the 2011 uprising which ousted Hosni Mubarak.
Reports said the four fatalities were all Brotherhood supporters who died amid fighting in two Cairo neighbourhoods.
As clashes broke out in the capital, state TV reported further violence in the northern Sharqiya district and to the east in Giza, as well as in the northern port city of Alexandria.
There were also reports of skirmishes between pro-Morsi demonstrators and civilian supporters of the military government.
The BBC’s correspondent in Cairo, Quentin Somerville, said that by early evening all was quiet on the streets of Cairo, ahead of the early Friday curfew at 1900 local time (1700 GMT).
A heavy security presence remained, he said.
Hundreds of Islamist protesters have died in violence since the Egyptian military deposed Mr Morsi in July, 13 months after he was elected as president.
Thousands of members of the Muslim Brotherhood have also been detained over the past two months.
Several senior figures, including Mr Morsi and the movement’s general guide Mohammed Badie, are being held on charges such as incitement to violence and murder.
Egyptian authorities portray the crackdown as a struggle against “terrorism”. They are preparing to seize Muslim Brotherhood assets after appeal courts upheld a recent ban on its activities.
Our correspondent says the protesters in the capital’s Agouza district were chanting “Rabaa, Rabaa”, a reference to the square next to the Rabaa al-Adawiya mosque where a sit-in was cleared by force in August.
Troops also took up positions on both sides of Qasr al-Nil Bridge, which leads to Tahrir Square.
As protesters pushed towards Tahrir, police and security forces used teargas and warning shots fired overhead to disperse the crowds.
Before Friday’s clashes, soldiers and police tightened security around key sites in Cairo, including Tahrir.
Morsi supporters have said they will be intensifying their demonstrations in the lead-up to Sunday’s 40th anniversary of the 1973 Arab-Israeli war.
Opponents who back the army have also said they will take to the streets.
In a statement issued on Thursday, the Brotherhood sharply criticised the officers behind the overthrow of Mr Morsi, comparing them to Adolf Hitler, the Roman emperor Nero and the Mongol conqueror Hulagu Khan.
It urged Egyptian soldiers to rebel and said it hoped that Sunday would mark a “victory by the people over those who staged a coup against them for personal gain”.
On Thursday, the European Union’s foreign policy chief, Baroness Catherine Ashton, held talks with armed forces chief Gen Abdul Fattah al-Sisi and interim President Adly Mansour, as well as with religious leaders.
“I got a real sense of everyone really trying to go forward in the right way,” she told reporters afterwards.
A day earlier, a 16-year-old boy was killed in clashes between Morsi supporters and opponents in the Red Sea city of Suez.
Photo: Many Morsi supporters used a four-finger salute, a symbol of the Rabaa al-Adawiya mosque where a sit-in was cleared in August
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