Jubilant Islamists across the Middle East hailed Mohammed Morsi's election as an electrifying victory for their cause, but there was mixed reaction from Arab governments as they struggled to gauge its geo-political ramifications.
There was sheer joy in the Gaza Strip, which is ruled by Hamas, an offshoot of Egypt's Muslim Brotherhood. Palestinians there fired off volleys of celebratory gunfire and handed out sweets in hopes Mr Morsi will end Cairo's cooperation with a punishing, six-year-old Israeli blockade.
"The Egyptian nation did not elect a president just for Egypt, but for the Arab and Islamic nations too," said Fawzi Barhoum, a Hamas spokesman.
There was no official response from Saudi Arabia, whose relations with the Muslim Brotherhood have been poor. Riyadh has often accused the Islamist movement of fuelling demands for political change in the kingdom.
Even so, analysts said Saudi Arabia would have to work with the new Egyptian president. "I think [the Saudis] are going to be very practical about it. More and more, they will discover common interests in the economy, in politics, on how to deal with Iran," said Jamal Khashoggi, a Saudi former newspaper editor.
A commentator for the Saudi-owned Asharq Al Awsat was more dramatic. "Be cautious and fasten your belts. Today the scenario many thought would not happen came true and will carry enormous repercussions," Tariq Al Homayad wrote in the London-based daily. "Egypt now stands at a serious crossroads that will have implications for Egypt and the region at large."
In contrast to Saudi Arabia, Qatar, which has a maverick foreign policy and long-time ties to the Muslim Brotherhood, was exultant over Mr Morsi's triumph. Since the beginning of the Egyptian uprising last year, Qatar has promised US$500 million (Dh1.83m) in aid to Cairo, while Qatari investors have promised more than $10bn in investment in Egyptian infrastructure.
Gerald Butt, the author of several books on the Middle East, said: "The Gulf states are generally unhappy with democratic change in a big country like Egypt because it increases the pressures from their own societies to introduce changes." But he added in an interview, "they won't be too worried in the short term because they know the Egyptian military is there as a stabilising force".
Hailing Morsi's victory, Iran paid tribute to the "martyrs of the Egyptian revolution", proclaiming they were responsible for ushering in a "splendid vision of democracy".
Some Arab commentators spoke of Arab states finding themselves caught between a Shiite Crescent and a Sunni Islamist arc.
Those concerns are likely exaggerated. Iran's image is on the wane in the region. It is under mounting pressure over its nuclear programme and has also grown more jittery over the turmoil in Syria, its only Arab ally.
Tehran's hopes of forging an opportunistic anti-American alliance with Egypt are far-fetched. Iran severed ties with Cairo a year after Iran's 1979 Islamic revolution because Egypt recognised Israel and gave asylum to Iran's ousted dictator, Shah Mohammad Reza Pahlavi.
But Egypt's Sunni Muslim Brotherhood is almost as mistrustful of non-Arab, Shiite Iran, as were previous secular Egyptian governments. And Cairo will be reluctant to jeopardise its lucrative relations with the Arabian Gulf nations by cosying up to Iran, alienating Washington in the process.
Senior Brotherhood figures have scoffed at Iran's claim that the Arab Spring uprisings are an anti-western "Islamic awakening" inspired by Iran's 1979 Islamic revolution.
They have even warned that Iran could be next in line for another pro-democracy uprising akin to the huge demonstrations against President Mahmoud Ahmadinejad's "stolen" re-election three years ago.
Equally embarrassing for Tehran, Syria's Muslim Brotherhood has branded Iran an "enemy" for allegedly arming President Bashar Al Assad's regime.
Catherine Al Talli, a Syrian human-rights lawyer, said events in Egypt were likely to encourage the Syrian opposition. "It shows that the popular will brings about democratic change, and that when the people chose to rise against repression, they can defeat it."
The opposition Syrian National Council agreed, calling Mr Morsi's victory "a source of hope for the rebellious Syrian people".
Islamists across the turbulent region saw his triumph through the prism of the Arab Spring uprisings that also toppled dictators in Tunisia, Libya and Yemen.
Mohammed Al Majidi, a protester in Yemen, told The National: "This is a victory for all the Arab Spring revolutions. It will inspire all people across the Arab world that change is possible and that this is the time of the people and no more to dictators and military rule."
Mohammed Al Qahtani, a Saudi pro-democracy activist, tweeted in agreement "This is a victory for the Arab revolutions".
Turkey, which sees itself as a role model for aspiring Islamic democracies, said Mr Morsi's win reflected the will of the people, but added he had a lot to prove. "Important tests await the new president who will lead the Egyptian people to the free and pluralist democracy they deserve," Turkey's foreign ministry said.
The Palestinian president, Mahmoud Abbas, a secular leader who governs in the West Bank, congratulated Mr Morsi and expressed his "respect for the choice of the great Egyptian people".
But Bassem Zubaidi, a political scientist at the West Bank's Birzeit University, feared an emboldened Hamas might now feel less willing to reconcile with the mainstream Palestinian Fatah movement. It could also "widen the gap between the Palestinians and Israelis reaching a constructive formula in peacemaking."