Egypt announced a criminal investigation on Saturday against deposed Islamist President Mohammed Morsi, with prosecutors saying they were examining complaints of spying, inciting violence and ruining the economy.

Egypt's first freely elected leader has been held at an undisclosed location since the army removed him from power on July 3, but has not yet been charged with any crime. In recent days Washington has called for him to be freed and for the authorities to stop arresting leaders of his Muslim Brotherhood.

The public prosecutor's office issued a statement saying it had received complaints against Morsi, eight other named Islamist figures including top Brotherhood leader Mohammed Badie, and others it did not identify.

The complaints are a first step in the criminal process, allowing prosecutors to begin an investigation that can lead to charges. Announcing the step was unusual: typically prosecutors wait until charges are filed before making public statements.

Badie and several other Brotherhood officials already face charges for inciting violence that were announced earlier this week, but most of them have not been arrested.

The prosecutors did not say who had made the complaints. Egyptian law allows them to investigate complaints from police or any member of the public.

Morsi's Brotherhood called on Saturday for more mass demonstrations after a huge march broke up peacefully before dawn, ending a week in which at least 90 people were killed.

Vigil

The Brotherhood, which has maintained a vigil near a Cairo mosque since before the army removed Morsi on July 3, has said it will not leave the streets until he is restored to power.

The military says it deposed Morsi in a justified response to popular demand after millions of people demonstrated against him. The Brotherhood says it was a coup that reversed democracy.

Turmoil in the most populous Arab state has alarmed Western donors. Egypt sits astride the Suez Canal and has a strategically important peace treaty with Israel.

Large crowds of Brotherhood supporters finally dispersed before dawn on Saturday after marching through the streets into the early hours holding up pictures of Morsi at traffic lights.

Tens of thousands had turned out on Friday for what the Brotherhood called a "day of marching on".

Morsi's opponents say those demonstrations are still much smaller than the ones that brought him down. However, the Brotherhood has shown its organizational muscle by keeping its vigil running into a third week and bringing in coachloads of supporters from the provinces during the Ramadan fasting month.

At one stage overnight, demonstrators stood behind barbed wire shouting at soldiers a few dozen meters away.

"I am here to say 'no' to the military coup and 'yes' to Morsi, who I see as my legitimate president," said Ahmed Adel, a 22-year-old student, in downtown Cairo.

Senior Brotherhood figure Essam El-Erian, one of those who face arrest, called on his Facebook page for more demonstrations on Monday.

"Egypt decides through the ballot box, through protests, mass marches and peaceful sit-ins," he said.