Egypt’s military leaders issued a constitutional decree Sunday that gave the armed forces sweeping powers and degraded the presidency to a subservient role, as the Muslim Brotherhood declared that its candidate had won the country’s presidential runoff election.

The bold assertion of power by the ruling generals followed months in which they had promised to cede authority to a new civilian government by the end of June. Instead, activists and political analysts said, the generals’ move marked the start of a military dictatorship, a sharp reversal from the promise of Egypt’s popular revolt last year.

The declaration, published in the state gazette, had been expected, but its details indicate that the military has asserted far greater authority than observers had anticipated. Under the order, the president will have no control over the military’s budget or leadership and will not be authorized to declare war without the consent of the ruling generals.

The document 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. Once a new charter is in place, a parliamentary election will be held to replace the Islamist-dominated lower house that was dissolved Thursday after the country’s high court ruled that one-third of the chamber’s members had been elected unlawfully.

“With this document, Egypt has completely left the realm of the Arab Spring and entered the realm of military dictatorship,” said Hossam Bahgat, a prominent human rights activist. “This is worse than our worst fears.”

The declaration left little doubt that the generals have moved aggressively to preserve and expand their privileged status after a transitional period that revealed the significant appeal of Islamist politicians. It also indicates the military leadership’s concern about accountability if a system of civilian rule with checks and balances were to take root.

The Obama administration, with the president spending the day in Chicago and much of his national security staff in Mexico preparing for this week’s Group of 20 summit there, had no initial reaction to the new developments. But the decree appeared likely to compound the administration’s frustration over its waning influence in Egypt.

Less than 48 hours before the declaration was issued, Defense Secretary Leon E. Panetta telephoned Field Marshal Mohammed Hussein Tantawi, head of Egypt’s ruling military council, to underscore “the need to ensure a full and peaceful transition to democracy,” the Pentagon said.

Thursday’s dissolution of parliament also prompted Sen. Patrick J. Leahy (D-Vt.), chairman of the appropriations subcommittee in charge of foreign aid, to warn the State Department against disbursing any of this year’s $1.3 billion in military aid to Egypt.

Brotherhood decries order

The Egyptian military’s declaration was issued just 20 minutes after the polls closed Sunday night at 10. The campaign of Mohamed Morsi, the Brotherhood’s candidate, said a limited sample of preliminary vote counts from across the country indicated that the Islamist leader was ahead of rival Ahmed Shafiq, who was the last prime minister appointed by Hosni Mubarak and was widely considered the generals’ preferred candidate.

Preliminary results published in the state-run Ahram Online news site showed that, with nearly 3.5 million ballots counted, Morsi was ahead with almost 55 percent of the vote. About 50 million Egyptians were eligible to cast ballots. Final results are expected Thursday.

But after hailing the preliminary results, Brotherhood officials decried the military’s declaration, calling it a stunning power grab.

“This is ridiculous, and it confirms that we’re facing a new dictatorship,” Mourad Mohammed Aly, a spokesman for the Morsi campaign, said in a phone interview.

The move comes after the country’s top judges, who were appointed by Mubarak, issued a ruling that dissolved the lower house of parliament, where the Brotherhood held nearly half the seats after elections last year.

The constitutional declaration will be binding at least until a new charter is approved. But because the generals will appoint the body that will draft that document, they are expected to ensure that the new constitution leaves them with continued power and shields them from scrutiny and prosecution.

After Mubarak’s ouster from the presidency, the generals portrayed themselves as champions of the revolution. But revolutionaries have since accused the military leaders of mishandling the transition and working to preserve their own interests. Last fall, in the face of a revolt against military rule that led to a security crackdown, the generals agreed to speed up the transition to civilian rule by holding a presidential vote no later than the end of this month.

“This makes it impossible to speak of a transfer to civilian rule at the end of June,” Heba Morayef, a Cairo-based researcher with Human Rights Watch, said Sunday night. “It was a soft coup to start with, but now it’s pretty blatant.”

Trading fraud allegations

Shafiq’s campaign issued a statement Sunday accusing the Brotherhood of “systemic” fraud, including ballot stuffing, voter bribing, voter intimidation and attacks near polling stations.

The Brotherhood’s violations, the statement said, prove that the Islamist group “does not believe in freedom of choice and democracy unless this democracy brings them to power.”

The statement said the Shafiq campaign filed more than 100 complaints of electoral violations with the presidential election commission. Aly, Morsi’s spokesman, rejected the charges, insisting instead that Shafiq’s camp had manipulated the vote.

Independent observers have not alleged large-scale fraud in the runoff vote, which was conducted Saturday and Sunday. But Sunday night’s allegations offered a taste of the contentiousness to come once a winner is declared.

The dissolution of parliament enraged Muslim Brotherhood leaders. The speaker of parliament, Mohamed Saad Katatny, issued a statement Sunday night after meeting with the generals that decried the appointment of a new panel to draft the charter.

Katatny reiterated that ­Brotherhood legislators intend to attend a scheduled session of parliament Tuesday, a move that could provoke a confrontation between the Islamist lawmakers and security forces.

Other prominent politicians called on Egyptians to resist the military’s actions. Abdel Moneim Aboul Fotouh, a former Brotherhood member who was a presidential candidate in the first round of voting, called the constitutional declaration a “full military coup” in a message on Twitter.

Mona El-Ghobashy, a political science professor at Barnard College, said the document puts the military beyond reproach. It is a role that the armed forces have taken for granted for decades because Egyptian leaders have hailed from the ranks of the military.

“The military stands over and above everyone else, elected by no one and unaccountable to anyone,” she said in an e-mailed statement.

Staff writer Karen DeYoung in Washington and special correspondent Ingy Hassieb in Cairo contributed to this report.