Islamist candidate Mohammed Morsi declared victory Monday in Egypt's first free presidential election since Hosni Mubarak's ouster 16 months ago.
But just as polls were closing, the ruling military council issued constitutional amendments that gave sweeping authority to maintain its grip on power and subordinate the nominal head of state.
After the last-minute power grab Sunday night, the ruling Supreme Council of the Armed Forces pledged Monday to honor its promise to hand over power to the newly elected president by the end of this month.
However, the constitutional amendments stripped the president of almost all significant powers.
The military decreed that it will have legislative authority after a court dissolved parliament, it will control the drafting a new constitution, and it will not allow civilian oversight of its significant economic interests or other affairs.
Mr. Morsi represents the Muslim Brotherhood, an Islamic fundamentalist group which has emerged as the most powerful political faction since the uprising.
The Brotherhood's Freedom and Justice Party rejected the constitutional declaration, saying it was no longer within the authority of the military council to issue such a decree with less than two weeks left for the transfer of power.
In Washington, Pentagon spokesman George Little urged the ruling military to transfer full power to a democratically elected civilian government, as it pledged to do in the past.
"We are deeply concerned about the new amendments to the constitutional declaration, including the timing of their announcement as polls were closing for the presidential election," said Mr. Little.
The constitutional declaration made almost simultaneously with polls Sunday night was the third major blow in a week to hopes for a democratic transition that arose from the uprising.
On Wednesday, the military gave itself broad powers to arrest civilians even on minor offenses such as traffic violations. And on Thursday, a court stacked with Mubarak-era appointees dissolved parliament.
Meanwhile, militants crossed from Egypt's turbulent Sinai desert into southern Israel on Monday and opened fire, killing an Israeli worker, defense officials said. Two assailants were killed in a gunbattle with Israeli troops responding to the ambush.
The Israeli was in a crew building a fence along the porous desert border.
No group claimed responsibility for the attack, which underscored the growing lawlessness in the Sinai desert since Mubarak was toppled by a popular uprising last year.
Military spokeswoman Lt. Col. Avital Leibovich said the assailants have not been identified but said defense officials suspect Palestinian militants in the Hamas-run Gaza Strip might have been involved.
Southern Gaza borders Sinai, and infiltrators can exit Gaza through tunnels under the border.
Several hours after the attack, an Israeli airstrike killed two men on a motorcycle in the northern Gaza Strip near the Israeli border. The Islamic Jihad said the men were members on a "reconnaissance" mission and vowed revenge.
Israeli military officials said the incident was not connected to the earlier infiltration from Egypt.
The mountainous Sinai desert harbors an array of militant groups, including Palestinian extremists and al Qaeda-inspired jihadists, Egyptian and Israeli security officials say.
Israeli Defense Minister Ehud Barak told Army Radio that there has been "a worrisome deterioration of Egyptian control" over the Sinai.