CAIRO — Egyptian President Mohamed Morsi forced out the country's two top military chiefs Sunday, in a bold move to wrest power from the armed forces and marginalize key holdovers of ousted President Hosni Mubarak's reign.

Seizing on public outrage over a brazen attack last week in north Sinai that killed 16 Egyptian security forces, Morsi on Sunday swore in a new defense minister, who will command the armed forces, and made additional major personnel moves. The president also announced that he had suspended a constitutional amendment the generals passed on the eve of Morsi's election giving themselves vast powers and weakening the presidency.

The ousted military chiefs quietly stepped aside Sunday, but analysts said the move could trigger a backlash and further polarize a nation in which many are wary of the intentions of the country's first Islamist president. Morsi ran as the candidate of the Muslim Brotherhood, an Islamist group that has yearned for decades to lead Egypt.

"This is a big moment of transformation in the history of Egypt," said Zeinab Abul-Magd, a history professor at the American University in Cairo who has studied the military closely. "Now, officially, it is a Brotherhood state."

Morsi's election in June was hailed as a watershed for a nation that for six decades had been governed by military autocrats. But efforts by members of the Supreme Council of the Armed Forces to cement their vast authority through legal maneuvers appeared to set the stage for a weak president.

The ouster of Field Marshal Mohammed Hussein Tantawi — the defense minister and top military chief — and his deputy, army chief of staff Sami Anan, suggested the Brotherhood is willing to act more quickly and assertively in taking control of key institutions than analysts had predicted.

"I want the armed forces to devote themselves to a mission that is holy to all of us, which is protecting the nation," Morsi said in a televised address.

Morsi on Sunday also appointed senior judge Mahmoud Mekki as his vice president, which may help with any legal challenges to the direction of Egypt's democratic transition.

The move appeared to catch U.S. Defense Secretary Leon Panetta and the Pentagon off guard. Panetta visited Egypt about two weeks ago and seemed to come away with the view that Tantawi and Morsi were cooperating. The United States has given Egypt billions of dollars in military aid in exchange for maintaining peace with Israel.

Morsi appointed Gen. Abdel Fattah el-Sissi as defense minister and commander of the armed forces, replacing Tantawi.

In June 2011, el-Sissi, who served as head of military intelligence and as a member of the supreme military council, gained attention when he told Amnesty International that "virginity tests" performed on female protesters during last year's revolution were meant to protect members of the military from accusations of rape. El-Sissi vowed such tests would no longer be conducted.

Muslim Brotherhood leaders called the president's moves a logical step for a new leader who has promised to make a fresh break with Egypt's authoritarian past. In recent weeks, Morsi visited military installations to share meals with low-level officers and soldiers. He also made complimentary statements about the armed forces.

"Mahmoud Hussein, secretary-general of the Muslim Brotherhood, said Sunday night that the moves came after a series of high-level meetings between the president and members of the supreme military council in response to the Aug. 5 attack in north Sinai. After ambushing an Egyptian checkpoint, the gunmen commandeered armored Egyptian military vehicles and rammed into an Israeli border crossing.

The assault gave Morsi the opportunity to make changes that otherwise would have almost certainly met strong resistance from members of the old guard, analysts and Brotherhood officials said.

It remained unclear Sunday whether some military members would object to the moves.

Suspending the constitutional addendum that gave the military unchecked power over defense issues puts the burden of governing squarely on Morsi's shoulders, after a monthlong period during which it was unclear who was really running the country.

When Morsi, a former political prisoner, took office last month and showed great reverence toward the country's military chiefs, Egyptians assumed the two sides were striving to strike a power-sharing deal. Many believed that the military, a historically secular institution, would rein in the Brotherhood if it tried to impose more religiously conservative social practices.

The swiftness with which the military's stalwarts were pushed aside left many Egyptians baffled, said Abul-Magd, the historian.

"There are no buffer zones whatsoever," she said. "Now it's just us and the Brotherhood."