Voters in Nigeria, Africa’s biggest country by population, go to the polls on Saturday to choose between the incumbent Muhammadu Buhari, his main rival, Atiku Abubakar and more than 70 other candidates.

Saturday’s election is seen as a referendum on Buhari’s first term, which has been marred by his prolonged absence due to illness, a weak economy, and the government’s failure to effectively tackle corruption and insecurity.

A faction of Boko Haram attacked a state governor’s convoy on Tuesday, killing four people and stealing vehicles. Elsewhere, 15 people were crushed to death at a ruling party rally in eastern Port Harcourt. On Thursday, 14 sacks of ballot papers were intercepted in Kano state – though police said they were merely “specimen” papers to educate voters.

Quick guide 2019 Nigerian elections Show Hide On Sunday 16 February, 84m voters will choose who leads Africa’s largest democracy. Despite over 60 candidates, it is likely to be a choice between President Muhammadu Buhari, and former vice president Atiku Abubakar. President Muhammadu Buhari (APC candidate) Buhari's 2015 victory with his All Progressives Congress (APC) party was built on three promises: to rid Nigeria of endemic corruption, fix the economy and tackle security threats. The government says it is making progress in the fight against Boko Haram, but it is now in its tenth year, and the economy entered and climbed out of recession under Buhari. Opponents say his government is failing to tackle corruption, while Amnesty International say the army has been responsible for human rights abuses. After spending five months in Britain in 2017 receiving treatment for an undisclosed ailment, opposition groups said he was unfit for office, but Buhari, 76, says he is strong enough to serve. Atiku Abubakar (PDP candidate) Main opposition People's Democratic Party (PDP) candidate Atiku, 72, has been caught up in corruption allegations since serving as vice president from 1999-2007. He has repeatedly denied any wrongdoing. He has promised business-friendly policies to double the economy to $900bn by 2025. He wants to privatise parts of the state oil company, and create a $25bn fund to support private sector investment. Atiku's opponents have claimed he would exploit those pro-business policies to enrich himself and those around him. A divided country Nigeria is deeply divided between the mainly Muslim north and largely Christian south. There are more than 200 distinct ethnic groups. This has led to an unofficial power-sharing agreement, with the presidency alternating between north and south every eight years. It isn't just religion or ethnicity causing fractures. Half of Nigeria’s registered voters are aged 18-35. Many say their ageing leaders are out of touch, and supported a "Not Too Young to Run" campaign to encourage younger people seeking office. Election concerns 2015 was the only time Nigeria had a peaceful power handover since civilian government took over in 1999. Even then there was evidence of vote buying, voter intimidation, and other forms of corruption. International observers fear election interference and rigging this time around. Last month, Buhari triggered a constitutional row when he suspended the chief judge, who has a crucial say in resolving election result disputes. Path to victory The candidate with the most votes is declared winner, as long as they have at least one-quarter of the vote in two-thirds of Nigeria's 36 states and the capital. Otherwise there is a run-off. Martin Belam and agencies Photograph: John MacDougall/AFP

Facebook Twitter Pinterest A supporter of Atiku Abubakar at an election rally in Yola, Nigeria. Photograph: Sunday Alamba/AP

Nigerian elections have a long history of rigging, and it is not usually limited to whichever side happens to be in power at the time. Powerful politicians move fluidly between the two main parties, taking support and manipulation mechanisms with them.

If free and fair, the election is expected to be very close, with many Nigerians either proclaiming themselves to be “Atikulated” and fed up with the state of the economy or chanting “Sai Buhari”, a term of respect used by those vigorously defending the president’s record.

In the ancient northern city of Kano, a Buhari stronghold, men in embroidered kaftan robes and colourful caps said they were voting for Buhari, while opposition billboards had been torn down in the streets. Innocent Okpanachi, an architect in Abuja, said: “The entire system is bad. You vote for who will unite the country.”

• This article was amended on 19 February 2019 to remove references to voter data registration figures. The original article quoted analysts who said there were indications that the data had been manipulated. Having been contacted by other analysts we now believe the data is not strong enough to support the original conclusion.