Muslim Brotherhood candidate Mohamed Morsi has been declared the winner of Egypt's presidential run-off, opening a new phase for the country following the toppling of Hosni Mubarak's government.

A huge cheer went up from Tahrir square, where supporters of Mr. Morsi gathered Sunday. The crowd celebrated with chants and fireworks — a big difference from nearly a year and a-half ago when Egyptians packed the square in anti-government protests.

Election officials say Mr. Morsi won nearly 52 percent of the vote, beating former prime minister and Mubarak-era official Ahmed Shafiq. Mr. Morsi becomes Egypt's first freely elected president.

Supporters of both Mr. Morsi and Mr. Shafiq had threatened a backlash should their candidate lose the closely contested election, but there were no reports of violence. Authorities deployed extra security around Cairo.

The United States issued a statement congratulating Mr. Morsi for his victory and urging him to advance national unity by “reaching out to all parties and constituencies” as he forms his new government.

Iran's foreign ministry and the Palestinian Authority also offered their separate congratulations to the region's newest elected leader.

Egypt's election commission was supposed to release the results Thursday, but said it needed more time to investigate fraud allegations against both candidates.

The office of the president emerges with fewer powers than the candidates expected when the army promised to hand over power to civilian rule by July 1.

The ruling Supreme Council of the Armed Forces recently carried out a series of moves aimed at solidifying its power, including the court-ordered dissolution of the Islamist-dominated parliament. The council also declared an interim constitution that gives its generals and the courts final say over much domestic and foreign policy, as well as the process to create a new, permanent constitution.