ABUJA, Nigeria - A former chemistry professor handpicked by President Olusegun Obasanjo won Nigeria's presidential election in a landslide yesterday, a vote denounced as deeply flawed by international observers and the opposition.

Umaru Yar'Adua must now fight for credibility in Nigeria, Africa's largest oil producer, where 15,000 people have died in civil strife and other violence since strict military rule ended in 1999.

Yar'Adua, 56, a Muslim from the north of a country of 140 million people nearly equally split between northern Muslims and southern Christians, has spent most of his working life in academia, teaching chemistry at a university in his home state.

Although he favors crisp Muslim robes and caps and presided over a state where Islamic law is practiced, he is not seen as a hard-liner or an especially strong supporter of Islamic law. Yar'Adua has vowed to follow the program of Obasanjo, a southern Christian, which includes privatization and opposition to spreading Islamic law outside the north or implementing stringent punishments, like amputations and death for adulterers, in the north.

Gen. Muhammadu Buhari, the 1980s-era military leader who ran second in Saturday's vote, called the vote "the most blatantly rigged election results ever produced in Nigeria."

During Saturday's presidential and parliamentary votes and a week earlier during elections for state governors and legislatures, electoral officials could be seen inking ballots and shoving them into boxes. Thugs intimidated voters. The presidential ballots lacked serial numbers, making them easy to mishandle and impossible to track.

In the United States, which counts on Nigeria as a top supplier of oil, the White House expressed concern about reports of election irregularities. "We hope that people can remain calm," White House deputy press secretary Dana Perino said.

State Department spokesman Sean McCormack said: "These were flawed elections and in some cases deeply flawed elections."

Yar'Adua rejected the rigging allegations.But he sought to reconcile with the opposition. "I have extended a hand of friendship to all my colleagues who contested with me," he said in a nationally televised news conference.

Oil prices rose yesterday, in part because of concern about Nigeria. Still, a widespread violent reaction appeared unlikely: Fear of security forces was likely to mute any response.

The electoral commission chairman, Maurice Iwu, said Yar'Adua won about 24.6 million votes, over three times those garnered by Buhari. About 61 million Nigerians registered to vote. Iwu gave no turnout figures.

In a nationwide address ahead of yesterday's announcement of the election results, Obasanjo accused the political opposition of "fanning the embers of hate" and engaging in "outright subversive activities."

Obasanjo, barred by term limits from running again, acknowledged that the vote was imperfect but said Nigerians were nonetheless devoted to democracy. He said the defeated should air grievances through the courts.

That was exactly the plan, said representatives of Vice President Atiku Abubakar, a onetime Obasanjo ally, who placed a distant third.