KABUL, Afghanistan - Hamid Karzai was declared the winner of Afghanistan's landmark presidential election today, after investigators concluded that a string of irregularities were too minor to overturn his triumph.

The country's joint U.N.-Afghan electoral board confirmed that the American-backed incumbent had clinched a five-year term as the country's first popularly chosen leader.

"His excellency Hamid Karzai is the winner of the election," board chairman Zakim Shah said at a ceremony in the capital. "We are announcing the first elected president of Afghanistan."

Shah said Karzai won 55.4 percent support in the Oct. 9 election, 39 points clear of his closest challenger and enough to avoid a second round.

A spokesman for Karzai, who was in the United Arab Emirates for the funeral of its late president, said his camp was "very glad to finally have the result we wanted" and appealed to rivals to put a bruising campaign behind them.

"We are starting a new life, a new Afghanistan and we hope everyone will cooperate with its reconstruction," Elmi said.

However, his nearest rival, former Education Minister Yunus Qanooni, refused to concede defeat, raising the risk of political instability in a country slowly emerging from a quarter-century of war.

In its final report released Wednesday, the election panel confirmed problems including ballot stuffing and with ink used to mark people's fingers to prevent multiple voting.

But it said there was "no evidence" that the problems were widespread, or that they favored only Karzai.