With about half of the results counted, Buhari leads with 8,520,436 votes compared to Jonathan's 6,488,210 votes. That's counting votes from 18 of 36 states, The Guardian reported.
According to the Independent National Election Commission, Buhari has won 10 states, including some in the south, where the president traditionally has enjoyed more support. Jonathan did win the capital territory of Abuja, though.
Should Buhari win the election, it will be the first time an incumbent president has lost in the history of Nigeria's democracy.
Supporters of opposition candidate Buhari held a protest in Rivers state in southern Nigeria, with gunfire and the arson of a polling place, claiming local election leaders conspired to compromise the fairness of Sunday's vote. Both he and incumbent President Jonathan signed a post-election "peace pact" pledging to avert violence, but each side was confident of victory.
A spokesman for Buhari's All People's Congress, Lai Mohammad, said, "There's been so much violence in rivers state that it's just not tenable. The ruling Peoples Democratic party called the election "credible."
There is a fear the losing party will not accept the results of the election. Over 800 people died in violence in northern Nigeria after the 2011 election. Each candidate used social media to send messages of calm.
The election was delayed for six weeks so the government could complete an offensive against Boko Haram insurgents. The results are expected Monday, but their announcement could be delayed.