Abuja, Nigeria ? A former chemistry professor hand-picked by President Olusegun Obasanjo won Nigeria’s presidential election in a landslide Monday, 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 some 15,000 people have died since strict military rule ended in 1999.

Yar’Adua, a 56-year-old 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.

Though 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 especially strong supporter of Islamic law.

Gen. Muhammadu Buhari, the 1980s-era military leader who was the runner-up in Saturday’s vote, called the outcome “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 bore no serial numbers, making them easy to mishandle and impossible to track.