CAIRO – The Muslim Brotherhood’s Mohammed Morsi was declared the winner of Egypt’s first free presidential election Sunday, and he proclaimed himself a leader “for all Egyptians,” although he faces a struggle for power with the country’s still-dominant military rulers.

The announcement by election officials touched off a joyous celebration of chanting and dancing by tens of thousands of Morsi’s supporters jamming Tahrir Square, the birthplace of the uprising that toppled Hosni Mubarak 16 months ago.

It also capped a week of growing political tension in the streets after authorities delayed announcing the results of the June 16-17 runoff election between Morsi and Mubarak’s former prime minister, Ahmed Shafiq.

Tanks and other signs of heavy security had been deployed around the country, especially outside state institutions, in anticipation of possible violence reminiscent of the first days of last year’s revolution.

President Obama telephoned the U.S.-educated Morsi to congratulate him on his victory and offer continued support for Egypt’s transition to democracy. The White House said Morsi expressed appreciation for Obama’s call and “welcomed U.S. support for Egypt’s transition.”

The reaction from Israel was subdued, with Prime Minister Benjamin Netanyahu saying he respected the results of Egypt’s democratic process and hoped the peace agreement between the two countries would remain intact. Ecstatic residents in the Hamas-ruled Gaza Strip filled the streets, fired guns in the air and handed out candy.

Speaking on Egyptian television Sunday evening, Morsi declared he had a “message of peace. We will respect all international agreements.” He did not mention Israel but the remark seemed to be a reassuring nod to respecting the peace treaty.

The election commission said Morsi won 51.7 percent in the runoff — a margin of only 800,000 votes — over Shafiq, a former air force colonel who was perceived to be the favorite of the military council that took over from Mubarak.

“I tell everybody in this memorable day, that because of your choice, your will, and after God’s favor, I am a president for all Egyptians,” the 60-year-old engineer, professor and former lawmaker said in his speech.

Monday’s editions of Freedom and Justice, the Muslim Brotherhood’s newspaper that bears the same name as the group’s political party, bannered the headline: “The street explodes with joy, the people write history: Morsi President of Egypt.”

It was a stunning victory for the Muslim Brotherhood, a group that was outlawed under Mubarak. But the liberal and secular youth groups that drove the uprising were left wondering whether Egypt has taken a step toward becoming a repressive Islamist state, or a new power sharing agreement between Morsi and the military — the traditional power brokers.

“This is not the best scenario I anticipated,” said Sarah Kamal, a liberal activist who was in Tahrir Square when Morsi’s victory was announced.

“I know they have sold the revolution short before. But they are better than the ‘felool,’ ” she said, referring to the remnants of the old regime. “I will stand with the Brotherhood against the military for now, and later I will fight off the Brotherhood’s hold.”