Violence has erupted in northern Nigerian cities as opposition parties have alleged rigging in the country's presidential vote. Hours earlier, Nigeria's electoral Commission announced incumbent President Goodluck Jonathan had won the nation's top job.

Nigeria's main opposition party has accused the ruling party of tampering with the results. They say irregularities ranged from voter intimidation to absence of accreditation of voters at most polling units.

Complete results had shown Nigeria's President Goodluck Jonathan had won another term.

A wave of violence swept across the northern part of the country.

Many people were killed over the last few days as Jonathan looked set for another victory. Property worth millions was destroyed in riots.

But international observers have said the election results were fair and credible.

They also called for calm and expressed hope judicial means could be used.

Alojz Peterle, EU election observer, said, "We know that all parties involved might not be pleased with the outcome of the elections but we would like to encourage all stake holders to use only legal and judicial ways to express their opinion or judgment or requests in case they think elections were not done in a fair way."

Meanwhile, the Nigerian army said it will defend the current democratic process in the country.

Colonel Mohammed Yerima, Defense Spokesman, said, "Nigerian armed forces remain committed and totally loyal to government of the federal republic of Nigeria, and as such would not sit down and watch unpatriotic people take laws into their hands, the military is solidly and fully committed to the protection of our democratic credentials and national security."

The rioters will be dealt with accordingly. But perhaps the political polarization between the Muslim north and the Christian south still remains a big issue to be solved.