The constitutional decree, published in the official state gazette, gives the military absolute control of all laws and the military budget, immunity from oversight and exclusive power to support or veto a declaration of war.
The document -- from ruling generals who for months had promised to cede authority to a new civilian government by the end of this month -- said the military would soon name a group of Egyptians to draft a new Constitution, which will be subject to a public referendum within three months.
State news media reported the ruling Supreme Council of the Armed Forces had already picked a 100-member panel to draft it.
Once a new charter is in place, a parliamentary election will be held to replace the Islamist-dominated lower house that the country's top court dissolved Thursday after ruling a third of the chamber's members had been elected unlawfully.
The announcement was issued 20 minutes after the polls closed Sunday night.
"With this document, Egypt has completely left the realm of the Arab Spring and entered the realm of military dictatorship," human-rights activist Hossam Bahgat was quoted by The Washington Post as saying. "This is worse than our worst fears."
The Obama administration, preparing for Monday's Group of 20 leaders summit in Mexico, had no immediate comment on the developments.
Two days before the declaration was issued, U.S. Defense Secretary Leon Panetta phoned council Chairman Field Marshal Mohammed Hussein Tantawi, Egypt's de facto ruler, to underscore "the need to ensure a full and peaceful transition to democracy," the Pentagon said.
U.S. Secretary of State Hillary Clinton called for a full transfer of power to elected civilians, telling reporters in Washington, "There can be no going back on the democratic transition called for by the Egyptian people."
The Muslim Brotherhood's political party claimed in a predawn news conference Monday its candidate, Mohamed Morsi, edged out Ahmed Shafiq, a former air force general who Hosni Mubarak appointed as prime minister 12 days before stepping down as president in the face of massive protests last year.
Shafiq has been widely considered the ruling generals' preferred candidate.
The brotherhood's Freedom and Justice Party claimed Morsi won 52.5 percent to Shafiq's 47.5 percent, with ballots from nearly all of the country's 13,000 polling stations counted.
Morsi thanked God for the victory, saying God "guided Egypt to this straight path, the path of freedom and democracy."
He pledged to represent all Egyptians, including those who had voted against him, and made a point to say he would support the rights of Egypt's Coptic Christian minority, many of whose members had rallied against him out of fear of the brotherhood.
Shafiq's campaign held a news conference to deny the Muslim Brotherhood's victory claim.
"We reject it completely," Shafiq campaign official Mahmoud Baraka told reporters.
"The campaign of Ahmed Shafiq is astonished by the conference of the FJP that represents a violation of the laws of the election commission," Baraka said, accusing the Muslim Brotherhood of "hijacking the election result."
Only the government can legally issue results, he said.
But "our counting of the votes have so far showed that we are ahead with 52 percent of the vote, but we refuse to break the law and issue any numbers now," he said.
The military's election commission said official results would likely be released by the end of the week.
Shafiq's campaign earlier alleged Muslim Brotherhood supporters committed voting irregularities, including tearing down Shafiq posters, bribing and intimidating voters, and "ballot rigging and stuffing."