Nigerian President Muhammadu Buhari has been re-elected for a second four-year term, final results from Saturday's general election show.The 76-year-old defeated his main rival, former Vice President Atiku Abubakar, with a margin of just under four million votes.Mr Abubakar's People's Democratic Party (PDP) has rejected the result.Delays and violence marred the run-up to the poll but no independent observer has cited electoral fraud.Mr Buhari's All Progressives Congress (APC) won in 19 of the 36 states while the PDP was victorious in 17 states and in the capital, Abuja, according to the electoral commission (Inec).Turnout was just below 35% of registered voters. The commission will make a formal declaration on Wednesday.

Who is Buhari?

A former soldier, Mr Buhari led a military regime for 20 months in the 1980s, and was first elected president in 2015, becoming the first opposition candidate to defeat an incumbent and win the presidency.His record in office is mixed. Mr Buhari's critics say that the very attributes that won over voters four years ago - his strictness and inflexibility - have emerged as liabilities. They accuse him of autocratic leanings as well as a disastrous tendency towards inaction.

• Profile: Muhammadu Buhari

Mr Buhari's supporters can argue that he has largely delivered on campaign pledges such as tackling corruption and cracking down on Boko Haram. But they may struggle to point to concrete achievements in other fields, such as fixing the economy.He will face a range of problems including power shortages, corruption, security threats, and an economic slowdown.The president has quelled a militant Islamist rebellion in Nigeria's north-east, but Boko Haram remains active. There has also been an upsurge in violence in the Middle Belt as traditional herders and more settled farmers have clashed.