CAIRO — Egypt’s newly elected Islamist president took a symbolic oath of office Friday afternoon in a packed Tahrir Square and delivered a rousing, defiant speech on the eve of an official ceremony at which the country’s ruling generals are set to nominally relinquish power.

Displaying uncharacteristic swagger, Mohamed Morsi, the long-shot Muslim Brotherhood politician who won the country’s first fair presidential vote, promised the ecstatic crowd that he would treat all Egyptians equally and remain accountable to those who elected him. His words underlined the sharp contrast between the low-key Islamist and ousted president Hosni Mubarak, whose autocratic style sparked a revolt in early 2011.

“You are the source of power,” Morsi told the supporters who had stood for hours under a searing sun to catch a glimpse of him. “You grant it, and you withdraw it.”

The message appeared designed to catch the attention of the generals, who will continue to wield vast authority even after Morsi assumes office on Saturday, by virtue of a constitutional decree the junta issued recently. Friday’s speech strongly suggested that Morsi intends to leverage his popular mandate against the military’s entrenched authority in coming months.

Mohamed Sayed, a 28-year-old engineer who voted for Morsi, said the new president faces myriad challenges, including asserting himself as a statesman atop a bureaucracy that includes many stalwarts of Mubarak’s dissolved political party.

“He will overcome these,” he said. “He is a president that is protected by the people and doesn’t wear a bulletproof vest.”

Another challenge will be bridging the societal gap his election exposed. Morsi was elected by a slim margin in the contest against a secular former air force chief whom many Egyptians favored, seeing him as better suited to restore order and prevent Islamist politicians from dominating the country’s power structures.