Egypt's new prime minister moved closer to forming a cabinet on Saturday, as prosecutors looked into criminal complaints against ousted Islamist president Mohamed Morsi and members of his Muslim Brotherhood.

Hazem al-Beblawi held talks with candidates for ministerial posts accompanied by vice president Mohamed ElBaradei, and the consultations will continue on Sunday.

Beblawi said he expected the cabinet to be made up of 30 ministries whose top priorities would be to restore security, ensure the flow of goods and services and prepare for parliamentary and presidential elections.

The premier is working according to a roadmap drafted by the military which toppled Morsi on July 3 after millions took to the streets calling on him to step down.

Morsi, the first freely elected president of Egypt, has not been seen in public since his ouster. Interim leaders say he is being held in a "safe place, for his own safety".

The public prosecutor's office said Saturday it was looking into complaints filed against Morsi and other members of the Islamist group including its general guide, Mohamed Badie, with a view to launching a formal investigation.

The complaints include spying, inciting violence and damaging the economy, although the prosecutor's office did not say who made the allegations.

During his single year of turbulent rule, Morsi was accused of concentrating power in the hands of the Muslim Brotherhood, sending the economy into freefall and failing to protect minorities.

His supporters say overthrow was a military coup and an affront to democracy, and refuse to join an interim government as tens of thousands have taken to the streets to demand his reinstatement.

"There will be another mass protest on Monday," Tareq al-Morsi, a Brotherhood spokesman, said on Saturday, a day after tens of thousands of Morsi supporters rallied in Cairo.

Protesters will also march on Monday to the Cairo headquarters of the elite Republican Guard, scene of deadly violence seven days earlier, the spokesman told AFP, insisting it would be "peaceful".

On Friday, Washington and Berlin called on the Egyptian military to release Morsi.

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

Psaki said Washington wanted "an end to restrictions on Mr Morsi's whereabouts", while Germany suggested the International Committee of the Red Cross should be granted access to him.

Rival demonstrators rallied in the capital on Friday, but while there had been fears of fresh violence, the evening passed off peacefully.

Tens of thousands of Islamist protesters gathered outside the Rabaa al-Adawiya mosque in Cairo's Nasr City to pray and break their fast together on the first weekend of the holy month of Ramadan.

Morsi supporters have been camping outside the mosque, where many Brotherhood leaders have been holed up, some wanted by authorities.

Firebrand Islamist preacher Safwat Hegazi, who is wanted by police for questioning on suspicion of incitement to violence, said the crowds would pursue their protests for "one or two months or even one or two years".

In a telephone call with Saudi King Abdullah on Friday, US President Barack Obama expressed "serious concern" about the violence since Morsi's overthrow and underscored the need to return to a democratically elected civilian government, the White House said.

The US sends around $1.5 billion a year to Egypt, most of it in military aid.

King Abdullah was the first foreign head of state to congratulate Egypt's interim president Adly Mansour, hours after he was named to replace Morsi.

Saudi Arabia and other Gulf Arab governments have pledged $12 billion in assistance to shore up the faltering economy.

In the worst single incident of the recent violence, clashes outside the Republican Guard headquarters killed 53 people on July 8, mostly Morsi supporters.

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

Arrest warrants have been issued for Brotherhood chief Badie and other top Islamist leaders suspected of inciting violence.

The military-appointed caretaker president has set a timetable for elections by early next year but Morsi opponents and supporters alike have criticised the interim charter he issued on Monday to replace the Islamist-drafted constitution.