Egyptian Islamist Mohammed Morsi has been declared the winner of the country's first free presidential election, and has promised to work for national unity and uphold international agreements.

In his first speech as president-elect on Sunday, Mr. Morsi offered a vision of inclusion — a sharp contrast to the polarizing campaign from which he emerged victorious against pro-establishment candidate Ahmed Shafiq.

Mr. Morsi called on all Egyptians, Muslims and Christians, to unite. He said this is the only way to get out of a difficult period since a popular uprising ousted longtime president Hosni Mubarak last year and left the military in charge of a chaotic transition.

Hours earlier, Egypt's election commission named Mr. Morsi the winner of a June 16-to-17 run-off election, with almost 52 percent of the vote, or a lead of more than 800,000 votes over Shafiq. Turnout was put at 51 percent. A huge crowd of Morsi supporters who filled Cairo's Tahrir Square erupted into celebrations, chanting victory slogans, waving flags and setting off fireworks into the night.

Mr. Morsi is the first civilian to be elected to the Egyptian presidency. The 60-year old U.S.-educated engineer ran for the post as a senior member of Egypt's main Islamist movement, the Muslim Brotherhood, a role that landed him in jail in the Mubarak era. After being confirmed as president-elect, Mr. Morsi resigned from the Brotherhood and its Freedom and Justice Party, in a gesture to other parties with whom he hopes to form a unity government.

In his speech, Mr. Morsi also pledged to respect all international agreements, saying Egypt “wants peace.” Egypt signed a peace treaty with Israel in 1979, becoming the first Arab nation to do so. The Brotherhood has been a strong critic of Israel and has refused to rule out revising the treaty.

Egyptian state news agency MENA said Mr. Morsi received congratulatory messages both from his defeated rival Shafiq and the head of Egypt's ruling military council, Field Marshal Hussein Tantawi.

The military council has promised to hand power to an elected president by the end of his month, but it also has made a series of declarations in recent days stripping the presidency of most of its powers. The council has taken for itself key executive powers and claimed control of legislative affairs after dissolving the Muslim Brotherhood-led lower house of parliament earlier this month.

The Brotherhood has rejected the military's newly gained powers, and Mr. Morsi's supporters have vowed to stay in Tahrir Square until those powers are surrendered, raising the prospect of a power struggle between the Islamists and the generals.

In Washington, the White House said President Barack Obama called Mr. Morsi to congratulate him on his victory. It said Mr. Obama pledged to continue to support Egypt's transition to democracy and to “stand by the Egyptian people as they fulfill the promise of their revolution.” The White House said Mr. Morsi welcomed U.S. support and agreed to work with Mr. Obama to advance the U.S.-Egypt partnership in the coming months.

In a separate statement, White House Press Secretary Jay Carney said the United States believes it is “essential for the Egyptian government to continue to fulfill Egypt's role as a pillar of regional peace, security and stability.”

U.N. Secretary-General Ban Ki-moon also congratulated Mr. Morsi. In a statement, Mr. Ban said he trusts that the incoming Egyptian president will “spare no effort in ensuring the Egyptian people “realize their aspirations for greater democracy, human rights and a more prosperous and stable” country. He said Egypt must “strengthen and build strong, independent institutions and allow civil society to flourish.”

The announcement of the election winner ended a tense week in which results were delayed as the commission examined complaints of fraud from both sides. Mr. Morsi and Shafiq both had claimed victory earlier in the week, and many saw the wait as a period of brinksmanship between the Brotherhood and the ruling military council over the post-election balance of power in Egypt.

Concerns remain about how Shafiq's supporters will react. Many expressed fear of growing Islamism in what has been one of the Arab world's more tolerant nations.