Abuja, Nigeria (CNN) -- Riots erupted Monday in cities across northern Nigeria as a spokesman for Goodluck Jonathan said he would emerge the victor in Sunday's presidential tally.

"He has won 25% in the necessary number of states and the majority vote -- he has satisfied the constitutional requirement to be president," said Ken Okolugbo, a member of the Goodluck Jonathan Campaign Council. Official results were expected Monday.

The Independent National Electoral Commission was posting on its website a running tally with results from 33 of the country's 36 states and its capital showing Jonathan far ahead of his main challenger -- Muhammadu Buhari.

Thousands of people participated in the outbreaks of unrest, which took place in Kaduna city and Zaria, in Kaduna state, and Kano. A witness who asked not to be identified out of fear of reprisal said a church was set afire in Kaduna, about two hours north of Abuja, the nation's capital.

Shehu Sani of the Civil Rights Congress said violence also erupted in Suleja, Niger state, where gunshots were heard, business owners shuttered their shops and military were on the streets. He described the scene as "pandemonium." Citing his sources there, he said there were reports of fatalities and injuries.

Sani said youths were targeting offices and officials of the ruling Peoples Democratic Party, and burned a house and a car belonging to an adviser to Vice President Namadi Sambo.

Jonathan's strength was in the south; in streets of the northern cities, rioters shouted Buhari's name.

On Sunday, riots occurred in Bauchi state, Adamawa state and Gombe state. A witness said tires were burned across Kano.

To avoid a runoff, Jonathan must get at least a quarter of the vote in two-thirds of the 36 states and the capital.

Nigerians voted Saturday for their president, a week after parliamentary elections were marred by violence and accusations of fraud in Africa's most populous nation.

Jonathan is the front-runner despite a poor performance in those elections by his People's Democratic Party. He is popular in the Christian and animist south.

The former vice president assumed office after President Umaru Yar'Adua died last year following treatment for a kidney ailment in Saudi Arabia.

Jonathan has led the nation of about 150 million people since May. About 73 million people were registered to vote.

His main challenger, Buhari, is a former military ruler and was a contestant in the 2003 and 2007 elections. He is the candidate for the Congress for Progressive Change and enjoys support from the mostly Muslim north.

Other candidates included Nuhu Ribadu and current Kano state Gov. Ibrahim Shekarau.

Saturday's voting was largely peaceful, in contrast to the violence that characterized the country's parliamentary elections on April 9. During that vote, separate bomb blasts ripped through a polling station and a collation center in northeastern Nigeria.

Human Rights Watch has estimated that at least 85 people have been killed in political violence so far.

A new election chief promised free and fair elections, but the electoral commission was forced to put off elections earlier this year by a week after logistical problems, including party logos missing from ballot papers, were reported nationwide.

Nigeria, Africa's most populous country and its largest oil producer, is a major supplier of crude oil to the United States, and it hosts many Western oil companies and workers.

Nigerians voted April 9 for 360 House of Representatives seats and 109 Senate seats. A gubernatorial vote will be held on April 26.

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