The official results of Nigeria’s presidential election show that opposition leader Mohammadu Buhari has a slight lead over incumbent President Goodluck Jonathan, as counting finishes in half of the states in the country.

Based on the results, which were released on Monday night, Buhari has so far won 8.5 million votes, compared to Jonathan’s 6.48.

Jonathan, however, has won nine states and the tiny Federal Capital Territory (FCT), compared to the nine states won by Buhari.

Votes from the remaining 18 states, including Lagos, which has the biggest number of voters, are expected to be sent to the counting center in the capital, Abuja, in the coming days.

A candidate needs to win over 50 percent of the total votes nationwide and take at least 25 percent of the vote in two-thirds of the states to be declared the final winner.

The ruling People’s Democratic Party (PDP) has governed Nigeria since the army relinquished power in 1999.

The tense vote presents, for the first time in Nigeria’s history, an opportunity for a challenger to defeat a sitting president.

Buhari’s knife-edge lead comes as he earned a massive victory in the key northern state of Kano.

He beat his rival by nearly 1.7 million votes in Kano, the state with the second-largest number of voters.

Kano is among the Nigerian states hit hardest by the acts of violence committed by Takfiri Boko Haram terrorists. Many have criticized Jonathan’s handling of the violence.

People in the oil-rich West African country turned out en masse to vote on Saturday. The elections were extended to Sunday, on the account that ballot papers arrived late or imported card readers failed to recognize the fingerprints of the voters.

The country’s 56.7 million registered voters also cast their ballots in parliamentary elections, with 739 candidates competing for a place in Nigeria’s 109-seat Senate and 1,780 seeking election to the 360-seat National Assembly during parliamentary elections.

The vote was marred by violence, with more than 40 people killed in attacks by Boko Haram terrorists in the northeastern state of Borno and another 14 killed in attacks in neighboring Gombe State.

Boko Haram, whose name means “Western education is forbidden,” had vowed to disrupt the polls by violent attacks.

The Takfiris have claimed responsibility for a number of deadly shooting attacks and bombings in various parts of Nigeria since the beginning of their militancy in 2009, which has so far left over 13,000 people dead and 1.5 million displaced.