Protests spread across Egypt as former PM agrees to form new government. Reuters

The White House has called for the Egyptian military to surrender all power immediately to a new civilian government that is due to be elected within the next few days.

In a marked increase of pressure on the ruling generals after days of hesitancy, Washington appears to have withdrawn its support for the army to retain a significant political role after next week's parliamentary elections.

But the US nevertheless threw its weight behind the ruling military council's plan to press ahead with the elections despite days of protest and dozens of deaths among demonstrators. Protesters have demanded the vote be postponed because the army would still have ultimate authority in running the country until the middle of next year.

The White House said the army council should swiftly cede full control to the newly appointed prime minister, Kamal Ganzouri, and his government.

"The United States strongly believes that the new Egyptian government must be empowered with real authority immediately. We believe that Egypt's transition to democracy must continue, with elections proceeding expeditiously, and all necessary measures taken to ensure security and prevent intimidation," the White House said in a statement.

"Most importantly, we believe that the full transfer of power to a civilian government must take place in a just and inclusive manner that responds to the legitimate aspirations of the Egyptian people, as soon as possible".

Previously, Washington had only condemned the violence in broad terms, angering many Egyptians by implying that the security forces and protesters were equally responsible.

It also previously urged the military to speed up the transition to civilian rule but did not publicly question the army's oversight of the process nor its plan to retain significant political power for many months, possibly years.

The White House statement marks a shift from its more ambiguous positions in which it sought to avoid publicly criticising the military regime which receives about $1.3bn a year in aid from the US.

In Cairo, more than 100,000 people packed into Tahrir Square on Friday for the biggest demonstration since the current showdown began, with activists accusing the generals of trying to extend the old guard.

Organisers called the demonstration "the last chance million-man protest." Swelling crowds chanted, "leave, leave" and "the people want to bring down the field marshal", in a reference to the military leader Mohamed Tantawi, who took over power from Mubarak.

Pro-reform leader and Nobel peace laureate Mohamed ElBaradei was mobbed by hundreds of supporters as he arrived in the square and took part in Friday prayers. The demonstrators have vowed not to leave the plaza until the generals hand power to a civilian presidential council.

Thousands of pro-democracy protesters took to the streets in other cities, including at least 10,000 in Alexandria and smaller crowds in Luxor and Assiut in southern Egypt.

About 5,000 supporters of the military staged their own demonstration in Cairo, several miles north of Tahrir in the district of Abbassiyah, not far from the defence ministry.

Tensions have risen ahead of parliamentary elections, which are due to begin on Monday. The elections are to be held in stages that end in March, and the military said on Friday it would extend the voting period to two days for each round in an apparent effort to boost turnout in response to the unrest.

Faced with this fresh wave of demonstrations, matching those which removed Hosni Mubarak from power after 30 years in February, Washington is concerned that the political crisis could further deteriorate and lead to the army reasserting full control or a political vacuum in which the Muslim Brotherhood wins power without the constraints imposed by the planned new constitution.

The White House would appear to be trying to avoid the mistake it made during revolution that broke out in January when it initially tried to keep Mubarak in office by pressing him to agree to cede some of his powers.

Some in Egypt regard the military government as a product of US machinations as it sought to ensure political stability and keep Islamists from taking power as Mubarak's rule collapsed. But the army is popular in Egypt, and it was widely welcomed when it took direct control of the government in February.

The White House said that since the start of the Arab Spring, the US has spoken out against violence and repression and "in defence of universal rights including the freedom of peaceful assembly". It says it supports for "political and economic reform that meets the legitimate aspirations of ordinary people throughout the region".

Washington is throwing its lot in with Ganzouri who on Friday said he has been assured by the army that he will have more power than that of the outgoing prime minister who resigned with his cabinet earlier this week as the latest wave of demonstrations centred on Cairo's Tahrir square grew.

Ganzouri, who was prime minister for three years in the 1990s under the ousted ruler, Hosni Mubarak, said he will wait until after the parliamentary elections to form a new cabinet. Ganzouri said that he is certain Tantawi is not intent on staying in power.

The White House said it is confident that the political crisis will be resolved. "Egypt has overcome challenges before and will do so again. The United States will continue to stand with the Egyptian people as they build a democracy worthy of Egypt's great history," it said.