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. His speech strongly suggested that he intends to leverage his popular mandate against the military’s entrenched authority.

In his first address before a large crowd since he was elected president, Morsi struck populist themes and engaged in theatrics.

In one crowd-pleasing gesture that will probably displease Washington but come to nothing, Morsi promised to fight for the release of Omar Abdul Rahman, an Egyptian serving a life sentence in the United States for his role in the 1992 World Trade Center bombing in New York.

He also paid homage to the protesters shot to death in and near Tahrir Square during the violent phases of the revolt.

At one point, the president-elect opened his jacket to show the crowd that he was not wearing a bulletproof vest, a dramatic gesture in a country where many remember the public assassination in 1981 of Mubarak’s predecessor, Anwar Sadat.