Kamal Ganzouri, who served as prime minister under President Hosni Mubarak, regained the post Friday as competing protests drew tens of thousands of demonstrators in the capital.
"We are here to serve our nation," said Ganzouri, who served as Mubarak's defense minister for 20 years.
Ganzouri told reporters that Hussein Tantawi, field marshal of the Supreme Council of the Armed Forces and Egypt's dominant force since Mubarak's ouster in February, "made it clear to me, he is no longer willing to stay in power. If he told me otherwise, I'd not have accepted to take this role."
Ganzouri said he had asked for time to form a cabinet "that will be accepted by everyone."
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Ganzouri took the job after Egypt's military rulers asked him to form a government of "national salvation."
The development came days after former Prime Minister Essam Sharaf and his government quit en masse and days before parliamentary elections scheduled to begin Monday. The high military council decided Friday to extend voting in the election to two days for each stage of the election, which will take several months, according to state-run Egypt TV.
"Of course, the new government cannot be formed before Monday's parliamentary elections," Ganzouri said.
Ganzouri, who was Egypt's prime minister between 1996 and 1999, is to remain prime minister until at least January 10, when results of the parliamentary elections are finalized, said Aly Hassan, a judicial consultant. After that, Parliament would have to back Ganzouri for him to retain the position.
Russian's Special Representative for Africa, Mikhail Margelov, had said any new prime minister would likely play a technical role, one that will require he not only run the government but "ensure the relevant political climate ahead of the elections," the Russian news service Interfax said.
The Alliance of the Revolutionaries of Egypt had proposed Mohamed ElBaradei, a 2005 Nobel Peace Prize winner and a frontrunner for the presidency of Egypt, to take over as prime minister, said coalition member Musad Ibrahim. He criticized the choice of Ganzouri, saying that he is 81 years old and asserting that "all his projects (in government) were failures."
"The security council wants someone they can control, and Ganzouri is their man," Ibrahim said.
Ganzouri takes the helm at a time of tremendous change, which he acknowledged in his news conference, saying that his new powers "are a lot more than I ever had ever before."
In Washington, the State Department tweeted that the new government "must be empowered with real authority immediately" and that the country's transition to democracy "must continue, with elections proceeding expeditiously, and all necessary measures taken to ensure security and prevent intimidation."
Several demonstrations in Egypt Friday could test whether the nation besieged by recent violent protests can remain peaceful after days of clashes.
By Friday, the death toll from the clashes stood at 41, including 33 in Cairo. An additional 3,250 have been wounded, said Dr. Hisham Shiha of Egypt's Health Ministry.
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He told CNN that many of the casualties had been shot by "live ammunition, rubber bullets and birdshot."
Friday's protests in Cairo's Tahrir Square have been dubbed "last chance."
There, tens of thousands of people massed Friday. Among them was ElBaradei, according to his Facebook page.
The alliance of protesters in the square "rejects the appointment of Ganzouri," a Nile TV reporter said. "They want a new name, a true national salvation government that doesn't include any old guard from the despot regime, and that the new cabinet has ministers who can represent them, their ages, their ambitions."
After five days of violence, the scene in Tahrir Square had calmed by Thursday, when soldiers erected barricades to separate protesters from police.
Demonstrators in the square have called for the interim military rulers to step down. The square was the epicenter of the movement that led to Mubarak's ouster as president nine months ago.
Also Friday, the Muslim Brotherhood was holding a "Victory to Jerusalem" demonstration, and a pro-military march started Friday afternoon in the upscale neighborhood of Abbasiya Square. There, supporters of the Supreme Council of the Armed Forces gathered in numbers smaller than those in Tahrir Square. Some held a banner that said, "To the defenders of the nation, we say Thank you." State-run Nile TV showed the crowd chanting, "The people and the military are ONE hand!" "Whoever loves Egypt doesn't destroy Egypt!" and "Enough! Enough! Let the people live!"
Meanwhile, Jehane Noujaim, an Egyptian-American documentary film producer who was arrested Wednesday, has been released, her lawyer Ragia Omram said Friday. Three American college students detained this week for allegedly throwing Molotov cocktails have been ordered released, but it was not clear when that order would be carried out.
Egypt's military leaders apologized Thursday for the deaths of protesters, vowing to prosecute offenders and pay the medical bills of the wounded.
"The Supreme Council of the Armed Forces presents its regrets and deep apologies for the deaths of martyrs among Egypt's loyal sons during the recent events in Tahrir Square," said the message, which was posted on the council's Facebook page. "The Supreme Council of the Armed Forces confirms that it is making every sincere effort to prevent such events from happening again."