Nigerian President Goodluck Jonathan was on course to win the country's elections on Monday, as official results showed him with a huge lead, but sporadic riots broke out in the north over rigging fears.

Millions of voters turned out for Saturday's election as Africa's most populous nation bid to put years of rigging and badly flawed ballots behind it and hold the cleanest polls for head of state in nearly two decades.

Observers gave the polls an initial thumbs up, but concerns were raised late Sunday over extraordinarily high totals favouring Jonathan in parts of his native south. Riots broke out in parts of the north over allegations of rigging.

Official results from Akwa Ibom state, for example, gave Jonathan 95 percent, while Anambra state figures showed him with 99 percent. His home state of Bayelsa gave him nearly a clean sweep -- 99.63 percent.

"Figures of 95 percent and above for one party suggest that these are fabricated figures and, personally, they worry me because they pose serious questions on the credibility of the election," said Jibrin Ibrahim of the Centre for Democracy and Development.

Jonathan's main challenger, ex-military ruler Muhammadu Buhari, won by significant margins in parts of the north, but nothing approaching such high figures in selected states for the incumbent.

Buhari had alleged reports of fraud on election day.