Cairo has witnessed another day of violence after supporters of ousted Islamist president Mohamed Morsi clashed with security forces after vowing a "day of anger" to protest the killing of at least 600 people on Wednesday.
In a confused situation around Egypt, officials said that at least 60 people had already been killed, including a police conscript ambushed at a checkpoint.
There have also been reports of at least 25 deaths outside the capital as the violence spread elsewhere in Egypt as the Muslim Brotherhood called for a week of protests.
The army and police force had earlier warned they would fire live rounds on civilians to protect state and army buildings from occupation. The country's interior ministry says police are allowed to use live ammunition "with a legal framework".
Tahrir Square, the hub of major protests throughout Egypt's major recent turmoil, was closed off and surrounded by armoured vehicles, and there have been clashes in the city's Ramses Square where foreign reporters have seen bodies and injured protesters.
"Despite the pain and sorrow over the loss of our martyrs, the latest coup makers' crime has increased our determination to end them," the Brotherhood said in a statement, announcing the march by millions of supporters on Friday after midday prayers.
"The struggle to overthrow this illegitimate regime is an obligation, an Islamic, national, moral, and human obligation which we will not steer away from until justice and freedom prevail, and until repression is conquered."
Thousands of people gathered in Ramses Square, with secularists joining Muslim Brotherhood supporters to oppose the conduct of police and armed forces.
Egypt's powerful military removed President Morsi from his post in July following popular demonstrations against his rule. He was arrested along with several senior members of the Muslim Brotherhood and its political wing, the Freedom and Justice Party. On Thursday, prosecutors extended his detention by another 30 days.
His ouster on 3 July was followed by several sit-in protests in Cairo, with the demonstration at the Rabaa al-Adawiya mosque being the focal point of the Islamists' protests. The use of deadly force by the military authorities to break up these protest camps has been the spark for this latest wave of violence.
Pictures posted on social media showed protesters writing their names in white marker on their arms, in case they are shot and need to be identified.
Reports on Friday indicated that shots were fired at protesters attempting to cross the May 15 bridge over the Zamalek neighborhood in Cairo.
People are jumping off the bridge pic.twitter.com/eknrUmn6Wx-- أبو كار (@Sarahcarr) August 16, 2013
Earlier, on 15 May Bridge, spoke to a man with a bandage over a bleeding head wound. Said police were shooting birdshot from rooftops.-- Jared Malsin (@jmalsin) August 16, 2013
Internationally, US president Barack Obama is facing growing criticism over his response to the crisis.
On Thursday, he cancelled a joint military exercise with Egypt, saying "our traditional cooperation cannot continue as usual", but gave no indication that the US would cut its £830 million (1.3 billion dollars) in aid to Egypt.
But Republican senator John McCain told BBC's Newsnight that the arrest of Mr Morsi had been a "coup", and US aid should be withdrawn as a result.
He said: "The law is very clear that if there is a coup that aid is cut off and we decided not to do that."
"It's a great tragedy and the United States is basically an observer," he added.
Europe's diplomats are to meet on Monday to discuss the situation, with some calling on the EU to freeze aid to Egypt.
In response to the violence and growing international unease, Egypt's government issued a statement pledging that it would work on restoring law and order and expressing "sadness" for the deaths of Egyptians, the Associated Press said.
But it said it was facing "terrorist actions targeting government and vital institutions" by "violent militant groups".
It also warned that Obama's position, while "not based on facts", could "empower the violent military groups".
The Egyptian ambassador to London - as did much of the state-controlled press in Egypt - blamed supporters of Mr Morsi for the outbreak of violence in Cairo, suggesting the large death toll was partly due to reckless protesters killing each other.
Speaking at a press conference in central London, Ashraf El-Kholy expressed his "deepest regret" at the loss of life, but said the police and army had been forced to shoot at demonstrators.
"Of course they did nothing but return fire," he said. "If you have somebody firing at you then you have to respond."