Thousands of supporters of the Muslim Brotherhood and deposed president Mohamed Morsi are heading towards downtown Cairo, chanting "down with military rule".
Egypt's military has bolstered its presence in the capital in anticipation of clashes after protesters called for a "day of anger", following the deaths of at least 638 people in a bloody crackdown on protest camps in Cairo.
Egypt is in a state of emergency and the police have been authorised to use live ammunition to protect themselves and key state institutions from attack.
The United Nations Security Council held an emergency meeting in New York about the crisis, the BBC reports.
Meanwhile, Egypt's interim leaders have criticised remarks by US President Barack Obama.
On Thursday, Mr Obama condemned the 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 $US1.3 billion 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".
The Muslim Brotherhood called on its supporters to gather in mosques for Friday prayers and then take to the streets of Cairo in a "march of anger".
The group's leaders said they would hold marches under the slogan "the people want to topple the coup".
Security in the capital is tight, with many armoured personnel carriers on the streets.
Members of groups opposed to Mr Morsi - the National Salvation Front and Tamarod - are reported to have called for counter-demonstrations in response.
There have also been calls for people to protect their neighbourhoods and churches throughout the country.
Egypt's Coptic Christian community has been targeted by some Islamists who accuse thecChurch of backing the army's overthrow of Mr Morsi last month.
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.
Authorities say the police were authorised to use live ammunition to protect themselves and key state institutions from attack.
