Egyptians picked a conservative Islamist as their first freely elected president, officials announced Sunday, giving the Muslim Brotherhood a platform to challenge entrenched military authority and electrifying the Arab world’s most populous nation with one of the most concrete signs of democratic change since the revolution last year.

Mohamed Morsi’s victory represented a watershed moment for Egyptian Islamists, who were tortured and repressed during decades of autocratic regimes, and it sparked jubilant celebrations in Tahrir Square, the heart of the revolution. But the result raised as many questions as it answered. Morsi will assume a position that was recently weakened by Egypt’s ruling generals through a constitutional decree. And he will not have the backing of the country’s Islamist-dominated parliament, which was dissolved by a court order.

It also remains uncertain whether the low-key, little-known Morsi can serve as a unifying figure in a nation that has splintered politically since the revolution, with many Egyptians fearful that Islamic leadership will impose strict moral codes or try to dominate politics. And though his win will serve as an inspiration for Islamist movements across the region, it is likely to be seen as a potential threat by Israel, which has regarded Egypt as a linchpin of Middle East peace through their 35-year-old treaty.

President Obama called the winner on Sunday and “emphasized his interest in working together with President-elect Morsi,” according to a U.S. statement. But the White House also said in a separate congratulatory announcement that it was “essential for the Egyptian government to continue to fulfill Egypt’s role as a pillar of regional peace, security and stability.”

In a televised victory speech Sunday night, Morsi cast himself as a leader for all Egyptians. He vowed to champion the rights of women and minority Christians, and he voiced conciliatory notes toward the armed forces. In what appeared to be an effort to demonstrate his independence, the Brotherhood announced Sunday that Morsi had resigned from the organization and its political party.

Egyptian president-elect Mohamed Morsi (The Washington Post)

“I will serve all Egypt. There will be no distinction between anybody,” a solemn Morsi said, standing behind a lectern emblazoned with the state insignia. “National unity is the only way to get Egypt out of this difficult time.”

Morsi, 60, who is viewed warily by many secular revolutionaries, paid tribute to the hundreds of Egyptians killed during the 18-day revolt last year that led to longtime president Hosni Mu­barak’s removal. Morsi promised the relatives of those slain that he would “not let their blood go to waste.”

In the short term, the election result is unlikely to curb the authority of the Supreme Council of the Armed Forces, or SCAF, which has ruled Egypt during the chaotic 17 months since Mubarak’s ouster. But it gives the Islamists a forceful injection of credibility, said Eric Trager, a fellow at the Washington Institute for Near East Policy, who was in Cairo on Sunday.

“While it is true that power remains with the SCAF, no one should underestimate the importance of the presidency as a bully pulpit for the Muslim Brotherhood,” said Trager, who studies the movement. Still, he added: “This will remain a fight between the Brotherhood and the military.”

The first battle is likely to come right away. The Brotherhood said Sunday that Morsi intended to take the oath of office before parliament, which was recently dissolved through a court order that the Brotherhood has denounced as illegitimate.

Mustafa Kamel el-Sayed, a professor of political science at Cairo University, said Morsi will have to fight to win back presidential powers that the military removed shortly after the runoff election a week ago and form a cabinet, which the generals are certain to want to influence. At the same time, Egyptians will be expecting him to address bread-and-butter issues, he said. Egypt’s economy has been battered by a drop in tourism, and crime has soared as Mubarak’s police state has eroded.

“This is a very positive development,” Sayed said of the election, “but the challenges are tremendous.”

Morsi’s supporters savored the victory with rambunctious celebrations across the country, expressing unbridled pride and relief after a tense week during which many Egyptians feared that the ruling generals were moving toward fraudulently proclaiming that Morsi’s rival, a pro-military candidate, had won.

Karim Ahmed, 23, was wrapped in a Morsi poster as people thronged into Tahrir Square, where they danced, cheered, ululated and clapped to the beat of drums and horns.

“Injustice can never come back,” Ahmed said. “This is the choice of the people!”

A senior U.S. official involved in Egypt policy said Washington grew alarmed last week as rumors swirled in Cairo that the military intended to coronate its favored candidate.

“We were concerned and said so,” said the official, who was not authorized to speak on the record. “In the end, the election authorities did the right thing and the votes prevailed. Now it’s time to focus on building a strong relationship with the new president.”

Morsi, an engineer, received a doctorate from the University of Southern California. He won a seat in parliament in 2000 and was a member of the Brotherhood’s shadowy Guidance Bureau, which sets policy, until he resigned last year to head the Brotherhood’s Freedom and Justice Party.

The often cantankerous politician originally was nominated as a backup candidate on the anticipation that Khariat el-Shater, a more revered Brotherhood figure, might be disqualified, which is what happened.

Morsi spent several months as a political prisoner for participating in protests after the 2005 parliamentary election and was detained during the early days of the 2011 revolt.

Morsi comes from the Brotherhood’s conservative wing, and during the campaign he vowed that he would make the Koran the bedrock of the new constitution. He has been critical of Israel’s treatment of the Palestinians but has not advocated for a repeal of the peace treaty.

In his speech Sunday, Morsi said he would “respect” Egypt’s international agreements but would also heed the will of the Egyptian people. The accord with Israel is deeply unpopular in Egypt.

Sen. John F. Kerry (D-Mass.) said Sunday that his recent conversations with Morsi in Cairo suggest the president-elect understands “the importance of Egypt’s post-revolutionary relationships with America and Israel,” adding that “actions will matter more than words.”

Stephen McInerney, executive director of the Washington-based Project on Middle East Democracy, said Brotherhood officials have not indicated an intention to scrap the treaty, which has made Egypt among the top recipients of U.S. aid since it was signed in 1979.

“You’ll see more inflammatory rhetoric and rhetoric demonstrating a willingness to stand up to Israel, but there does not appear to be a desire to change the fundamental points of the peace treaty,” McInerney said.

Election officials said Morsi captured almost 52 percent of the votes in a runoff against former Mubarak prime minister Ahmed Shafiq.

Even those who weren’t gladdened by Morsi’s victory said they were nonetheless pleased that the country had had a free vote.

“It’s a reality now,” said Mohammed Said, 32, a flag vendor in Tahrir.