The winner of Nigeria's presidential election, Muhammadu Buhari, has been sworn in as leader of world's most populous black country, after former president Goodluck Jonathan urged his successor to unite the country in the face of the threat from Boko Haram militants.

Buhari, the first opposition leader to win a Nigerian presidential election since independence in 1960, unseated Jonathan - who had been in office since 2010 - by more than two million votes after pledging to tackle Boko Haram, an Islamist insurgency that has been causing havoc in the north-east, and stamp out endemic corruption, which in the past has marred almost every administration.

Buhari, 72, a military ruler-turned-democrat, took the helm of Africa's biggest economy on Friday 29 May, 2015 at the inauguration ceremony at the capital's Abuja Eagle Square, where Jonathan handed over the constitution and national flags before Mr Buhari took his oath of office.

US Secretary of State John Kerry attended the ceremony alongside African leaders including Zimbabwe's President Robert Mugabe, South Africa's President Jacob Zuma and Mali's President Ibrahim Boubacar Keita.

Political sabotage

Buhari, who will be moving into the Presidential Villa, Aso Rock villa, has promised his countrymen a more democratic approach during his second time in office.

Jonathan called Buhari to congratulate him, conceding defeat as the results emerged in March, but Buhari's All Progressives Congress (APC) has accused outgoing officials of creating an "atmosphere of contrived chaos" to sully Buhari's political success, referring to weeks of strike action by fuel distributors which have crippled the economy.

In a statement, APC spokesman Lai Mohammed said the "whole scenario reeks of sabotage". The strike has ended after causing massive disruption, as flights were cancelled, banks closed and phone signals cut. Fuel distributors claimed they were being owed $1bn by the government.

Who is Muhammadu Buhari?

- He is 72 and hails from Daura in the predominant Muslim North

- An authoritarian military leader during 1984-85 when he seized power in a coup

- He has been candid about his conversion from dictator to democrat

- He has run for the country's top job three times before his election, including in 2007 and 2011

- He headed the People's Democratic Party (PDP) until setting up the All People's Congress (APC) in 2013. The PDP has been haemorrhaging support since his departure as APC has attracted PDP defectors and gained momentum

- He survived an assassination attempt in July 2014

- He has been an outspoken critic of Jonathan's perceived inaction over Boko Haram