CAIRO (AFP) - The United States (US) on Friday called on Egypt's military to free deposed president Mohamed Mursi, as tens of thousands of his supporters vowed to keep fighting for his reinstatement.

State Department spokeswoman Jen Psaki said US agreed with Germany's earlier appeal for Mursi to be released and was "publicly" making the same request.

The deposed Islamist president has been held in a "safe place," according to Egypt's interim leaders, and has not been seen in public since his ouster on July 3.

Ms Psaki said US officials had been in regular contact with all sectors of Egyptian society, and Washington was echoing German calls for "an end to restrictions on Mr Mursi's whereabouts".

A German foreign ministry spokesman said a "trusted institution" such as the International Committee of the Red Cross should be granted access to Mursi.

As night fell on Cairo, tens of thousands of Islamist protesters prayed and broke their fast together on the first weekend of the holy month of Ramadan.

They had spent the day protesting outside the Rabaa al-Adawiya mosque in the Nasr City neighbourhood, holding Egyptian flags and Qurans, chanting against the military coup that unseated Egypt's first freely elected president.

"We are here to deliver a message to the military that we won't give up on legitimacy. We will fight for our rights," said protester Ashraf Fangari.

"We are here to defend our votes. They were stolen from us." Millions of Egyptians had taken to the streets to demand Mursi's resignation - accusing him of being a puppet of the Muslim Brotherhood and of failing to fulfil the people's aspirations of freedom and social justice.

The mass anti-Mursi demonstrations prompted an army intervention and days of bloody clashes.

"We will continue to resist. We will stay one or two months, or even one or two years. We won't leave here until our president, Mohamed Mursi, comes back," key Islamist leader Safwat Hegazi told Friday's crowd.

Mr Hegazi demanded Mursi's reinstatement, immediate parliamentary elections and a committee to oversee a plan for national reconciliation.

Thousands also massed in support of Mursi outside the University of Cairo, watched over by a heavy security presence.

Despite the defiance, the interim authorities are pressing ahead with forming a new government amid financial help from Gulf states to shore up the faltering economy.

Interim premier Hazem al-Beblawi has decided on 90 per cent of his proposed cabinet line-up and would begin talks on Saturday with the nominees, sources told the official Mena news agency.

The ministers of defence and interior in the previous government are due to keep their jobs and that the full cabinet line-up would be finalised by the middle of next week.

The Muslim Brotherhood, which is now in tatters, its leadership detained, on the run or keeping a low profile following Mursi's overthrow, said it will not join the new government.

The onset of the holy fasting month of Ramadan, usually a time of communal sharing and unity, has been marked by tensions after deadly clashes.

Rival demonstrators also rallied in the capital on Friday.

In Cairo's Tahrir Square and outside the Ittihadiya presidential palace, hundreds of anti-Mursi protesters sat down for a festive iftar meal to break the dawn-to-dusk Ramadan fast.

The rallies have raised fears of more of the violence that has shaken Egypt since the army removed Morsi.

In the worst single incident, clashes at an army building in Cairo on Monday killed 53 people, mostly Mursi supporters.

The Brotherhood accuses the army of "massacring" its supporters, while the army says soldiers were attacked by "terrorists" and armed protesters.

On Friday, gunmen in the Suez Canal city of Ismailia killed a police conscript and wounded an officer when they tried to stop a vehicle the armed group was travelling in, Mena said.

The restive Sinai peninsula, home to Egypt's luxury Red Sea resorts, has been hit by a surge of violence, with militants killing a police officer early on Friday, officials said, and a Coptic Christian man found decapitated a day earlier.

On Wednesday, two people died in an attack on a checkpoint in the Sinai.

Police are hunting Brotherhood chief Mohammed Badie and other senior leaders suspected of inciting violence, after arrest warrants were issued on Wednesday.

And the public prosecutor has pressed charges against 200 of 650 people detained during Monday's violence.

Mr Adly Mansour, the military-appointed caretaker president, has set a timetable for elections by early next year.

But Mursi opponents and supporters alike have criticised the interim charter he issued on Monday to replace the Islamist-drafted constitution and steer a transition that the army itself has acknowledged will be "difficult".

Many fear a repetition of the mistakes of the last military-led transition, between Hosni Mubarak's ouster in 2011 and Mursi's election in June 2012.