Kaduna , Nigeria ( CNN ) -- Nigerians voted for president Saturday , a week after parliamentary elections were marred by violence and accusations of fraud in Africa 's most populous nation . Incumbent Goodluck Jonathan is the presidential front-runner despite a poor performance in the parliamentary and senate elections by his People 's Democratic Party . 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 are registered to vote . His main challenger -- former military ruler Muhammadu Buhari -- was a contestant in the past elections in 2003 and 2007 . Buhari is the candidate for the Congress for Progressive Change . Other candidates include Nuhu Ribadu and current Kano state Gov . Ibrahim Shekarau . A CNN iReporter in Lagos , who gave her name as Jan Young , said she expected the race to be tight . `` If the incumbent president wins , it wo n't be a landslide victory but a fair split between the ruling and opposition parties who campaigned for our votes . I also expect our nation would demand accountability from whoever wins at the end of the day , '' she wrote . The winner will face tough challenges in the former British colony that has been mostly under military rule in its 50 years of independence . For starters , to avoid a runoff , the winner must get at least a quarter of the vote in two-thirds of the 36 states and the capital . The ruling party has a tradition of alternating power between the mostly Muslim north and the Christian and animist south after two terms in office . The rhythm was broken when Jonathan , a southerner , succeeded Yar'Adua during his second term in office . Some ruling party leaders have demanded a candidate from the north . Saturday 's voting was largely peaceful , except for a bomb that exploded in northeastern Nigeria early in the day . The blast was in Maiduguri city in Borno state , according to a spokesman for the National Emergency Management Agency . Gunfire also rang out in that city , the spokesman said , but there were no immediate reports of casualties . The relative calm stood in contrast to the violence that marred the country 's parliamentary elections last week . 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 . It was a major setback reminiscent of the nation 's 2007 elections , which the European Union described as filled with rampant vote rigging , violence , theft of ballot boxes and intimidation . Nigeria , Africa 's most populous country and its largest oil producer , is a major supplier of crude oil to the United States , and hosts many Western oil companies and workers . Nigerians voted April 9 for 360 House of Representatives seats and 109 Senate seats . After the presidential election on Saturday , a gubernatorial vote will be held on April 26 . Nigerians are working to harness the power of the internet this year to demand accountability and encourage citizens to get involved in the voting process . The west African nation has 43 million internet users -- the largest in Africa -- and they are increasingly using Twitter , Facebook and blogs to report cases of voting problems during the election season .
The voting was largely peaceful . President Paul Jonathan is the front - runner . His victory comes despite a poor showing in the parliamentary vote last year . His main challenger is former military ruler Prince Mohammed .