Abuja, Nigeria(CNN) Nigeria's President Muhammadu Buhari has been declared winner of the country's elections, according to the electoral commission who said he polled 56 percent of the votes.
Buhari, who first came to power in 2015 after defeating then-incumbent Goodluck Jonathan, defeated his main challenger Atiku Abubakar by a margin of nearly four million votes in results announced by the Independent Electoral Commission (INEC), in Abuja, Nigeria's capital.
Buhari of the All Progressives Congress (APC) claimed 15,191,847 (56%) while Abubakar received 11,262,978 (41%) of votes in the weekend election, according to the commission, which finished counting state-by-state results on Tuesday.
"The new administration will intensify its efforts in security, restructuring the economy and fighting corruption," Buhari said after his victory was officially announced.
Abubakar, of the People's Democratic Party, (PDP) rejected the election results calling the electoral process "militarized" and a "disservice" to Nigeria's democracy.
"If I had lost in a free and fair election, I would have called the victor within seconds of my being aware of his victory to offer not just my congratulations, but my services to help unite Nigeria by being a bridge between the North and the South," Abubakar said.
He added: "I hereby reject the result of the February 23, 2019 sham election and will be challenging it in court."
Abubakar, who was Vice President from 1999 to 2007, has run for President four times. His candidacy was dogged by allegations of corruption stemming from his time in office, allegations he has consistently denied.
Buhari won in 19 of the 36 states with Atiku gaining the upper hand in 17 states and in the capital, Abuja.
Delivering Africa's largest democracy was never going to be an easy task, but Nigeria's elections were marred by poor logistics arrangements, which led to the electoral commission moving the presidential election date by one week -- just hours before the polls opened.
On the day itself, there were reports of voter intimidation, ballot boxes snatched at polling stations and voting papers burned in at least one place. Nigerians were left waiting for several hours at polling units across the country for voting materials to turn up.
Consequently, there was widespread voting apathy, leading to Nigeria's lowest voter turnout in 20 years, with INEC reporting that just 27 million votes were cast out of 84 million people who registered to vote.
In Lagos, the sprawling megacity of nearly 20 million inhabitants, only 1.1 million people voted, according to INEC figures.
The election turnout was 35.6 percent, the electoral commission said, which compared with 44 percent in the 2015 elections.
At least 39 people were killed in election-related violence, according to the Situation Room, a coalition of more than 70 civic organizations that monitored the elections. One of those who died was Ibisaki Amachree, a mother of two, an election volunteer caught in the crossfire between the Nigerian army and armed militia in Rivers State.
The Situation Room, citing data from analysis firm SBM Intelligence, said most of the deaths occurred in the southern oil-rich state.
But despite the violence, US observers from the International Republican Institute and the National Democratic Institute (IRI/NDI) said that although the deaths were tragic, they should be placed in the broader context.
"We don't want to overemphasize the deaths," the IRI's John Tomaszewski told CNN Monday.
"We understand the contextual relationship. Violence happens every day here. It's not just on Election Day."
The death toll reported in the presidential vote is lower than in previous national elections. The International Crisis Group said at least 100 people were killed in violence that broke out during and after the 2015 election and Human Rights Watch reported 800 killed in the post-election violence of 2011.
Buhari first came to power as a military leader in a coup in 1983 before being ousted himself 20 months later in another coup.
He ran unsuccessfully for President three times from 2003 to 2011, and was elected into office four years ago as the first opposition candidate to defeat an incumbent in Nigeria.
Buhari swept to power promising to tackle endemic corruption and root out the terror group Boko Haram.
Although Buhari is perceived as having a zero tolerance reputation in his fight against corruption, Transparency International said Nigeria has "neither improved nor progressed in the perception of corruption in the public administration in 2018," in its latest Corruptions Perception Index.
The fight against Boko Haram also seems to have stalled as the jihadi group appears strengthened and one faction has forged stronger links with ISIS to create the Islamic State of West Africa Province, and the group is staging attacks on military bases and displacing thousands of Nigerians.
Buhari is now faced with the daunting task of reviving a flagging economy that has been affected by the global oil price crash from its $100 high to $40, leaving the country's major revenue source depleted.
Although the country has emerged from recession and GDP grew slightly, 91 million Nigerians now live in poverty, the highest number of impoverished people in the world, overtaking India to move into the top spot in 2018, according to The Brookings Institution.
Buhari will also need to show the Nigerian people that he has put his health problems behind him.
From May 2016 until mid-2017, Buhari visited the UK for medical treatment; it prompted fake news reports that he had died in London and he was forced to deny claims that he died and was replaced by a clone.