CAIRO — The first celebrations for the new president were joyous but spare, a party overwhelmed by traffic and heat. In Tahrir Square, flags waved and boys set off fireworks that disappeared in the midday sun.

The hundreds who gathered on Monday cheered for Mohamed Morsi, a candidate of the Muslim Brotherhood who had held off a challenge from Hosni Mubarak’s former prime minister and is set to become Egypt’s first freely elected president. A milestone had passed and victory had been declared. But even among the revelers, doubts remained.

“I didn’t sleep last night. I am happy we got rid of a remnant,” said Ahmed Adel, speaking of the defeated candidate, Ahmed Shafik. On Monday, Mr. Adel was forced to come to terms with Mr. Morsi, who had not been his first choice, or even his second. “No one knows him,” he said.

Wrung through a never-ending transition, Egyptians spent the last few days reeling from new tests. Voters, faced with polarizing candidates at the polls, struggled with their choices. The country’s military rulers issued new edicts, dissolving Parliament and stripping the presidency of much of its power.

On Monday, Egyptians puzzled over their newly elected leader, whose accidental candidacy thrust him to the center of a tense national debate about citizenship, religion and politics. As president, Mr. Morsi, a symbol of Egypt’s divides, was suddenly responsible for healing them.

“People who voted for Morsi chose him because of the institution he represents, not because of him,” said Nermine Gohar, a 39-year-old homemaker, one of many people — critics and supporters — who felt that the Brotherhood and its candidate were inseparable. “Morsi and the Muslim Brotherhood have earned this. They worked harder and longer than anyone else.”

Mr. Morsi had not even been supposed to run. After the 2011 Egyptian revolt, the Muslim Brotherhood declared it would not field a candidate in the race, with officials saying they feared the public would think them too eager to grab power.

After the group changed its position — claiming it had not been able to find an independent candidate it could work with — Mr. Morsi, 60, became their standard-bearer, but only after the Brotherhood’s first choice, Khairat al-Shater, the group’s leading strategist, was deemed ineligible.

During the first round of presidential voting in May, Mr. Morsi distinguished himself in a crowded field of candidates for his lack of charisma and for espousing polarizing, conservative views.

He barely appeared in his own campaign advertisements. His early campaign stops with Mr. Shater led to the frequent charges that Mr. Morsi was simply a stalking horse for the disqualified leader. People took to calling him the “spare tire.”

Decades ago as a graduate student in the United States, Mr. Morsi also seemed like an unlikely Islamist leader, said Farghalli A. Mohammed, a friend and former professor. Pursuing his doctorate in materials sciences at the University of Southern California in the 1980s, Mr. Morsi was a bright student, and keen to socialize, but not especially pious or dogmatic. With a thesis titled “High-Temperature Electrical Conductivity Structure of DonorDoped Alpha-Aluminum Oxide,” he went on to become a professor.

“I never thought he would be a person associated with the Muslim Brotherhood,” Dr. Mohammed said.

Mr. Morsi became one of the group’s most prominent leaders, serving as the head of the Brotherhood’s parliamentary bloc and on its guidance council. He developed a reputation as a conservative enforcer, stamping out political dissent within the group.

In May, after winning the first round of the presidential vote, Mr. Morsi found himself facing Mr. Shafik, a former civil aviation minister. As Mr. Shafik tried to amplify the threat posed by the Muslim Brotherhood, Mr. Morsi tacked to the center. The Brotherhood issued statements saying it would have no problem with a female or Christian vice president, although Mr. Morsi indicated he would not support women or Coptic Christians as president.

His victory would inaugurate Egypt’s first experiment in Islamist governance. For decades, the government made dire warnings about the Islamists, outlawing the Brotherhood and imprisoning its members, while seizing on the threat to justify one-party rule.

On Monday, Ms. Gohar said she was sure the warnings had been overblown. “There is this ignorant fear that the Muslim Brotherhood will turn our country into Iran,” she said. “We need to give Morsi a chance.”

Others were more alarmed. “Mubarak was not kidding when he said that it was either him or the beards,” said Youssef Tamam, 36. “What do we even know about this Morsi? Everyone knows he won’t be calling the shots. His own organization didn’t think he was the man for the job.”

Hany Essam, 42, a driver, spoke darkly of Egypt’s turning into Saudi Arabia. “We went to Tahrir for freedom, and now we will be run by an even more oppressive state,” he said. “What a failure this revolution was.”

A few blocks from Tahrir Square, Ahmed Abdelradi, a petroleum worker from Suez, sat in a sidewalk cafe explaining how he voted for Mr. Morsi to “protect the revolution.”

Reflecting on the draining campaign, the long road Egyptians had been forced to travel and the uncertainty ahead, he said: “We just want the country to calm down. Strange things are happening. Strange things.” Liam Stack and Dina Salah Amer contributed reporting. (Copyright 2012 The New York Times Company 06/19/12)