Abuja - Muhammadu Buhari was sworn in as Nigeria's new head of state on Friday in a ceremony mixing military pomp with cultural tradition after he won the first opposition victory over a sitting president in the nation's history.
The 72-year-old took the oath of office before the Chief Justice of Nigeria, Supreme Court President Mahmoud Mohammed, to begin a four-year term in which he faces difficulties from the outset.
"I, Muhammadu Buhari, do solemnly swear that I will be faithful and bear true allegiance to the Federal Republic of Nigeria," he said at the ceremony at Abuja's Eagle Square venue.
"That as President of the Federal Republic of Nigeria, I will discharge my duties to the best of my ability and in accordance with the constitution of the Federal Republic of Nigeria and the law."
The former military general inherits a country facing crises on several fronts, from severe economic turmoil to Boko Haram's still-raging Islamist insurgency.
Buhari's inauguration, before past Nigerian leaders and serving heads of state from across the continent, comes 32 years after he seized power in a military coup and was ousted 20 months later.
Before taking the oath, he shook hands with the elected president he ousted in 1983, Shehu Shagari, and the general who deposed him, Ibrahim Babangida.
Buhari has described himself as a "converted democrat" and vowed to lead an administration committed to the needs of Nigeria's 173 million people by cracking down on the scourge of corruption.
But analysts said his first task may be managing worldwide expectations, given that the country has struggled for decades with woeful infrastructure, crippling unemployment and widespread unrest.
But beyond the political challenges facing the new government, the historic importance of the ceremony should not be overlooked, said Clement Nwankwo, an activist who fought against military rule.
"The handover... represents a significant milestone in the democratic development of Nigeria," Nwankwo, who now heads the Policy and Legal Advocacy Center, told AFP.
Civilian rule was restored in 1999 but the Peoples Democratic Party (PDP) has held power at the federal level for the last 16 years, at times appearing intent on staying on at any cost.
Buhari's win over outgoing president Goodluck Jonathan in March 28 polls and the subsequent inauguration of a long-time opposition leader were not expected, added Nwankwo.
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Corruption, economic crisis
Plunging oil prices have caused a cash crunch that has left thousands of civil servants unpaid and pushed the naira currency to historic lows.
Buhari and his All Progressives Congress Party have pledged sweeping change, particularly in job creation, electricity supply and insecurity.
But with the public coffers in tatters, Buhari's ability to deliver on campaign promises may be limited in the short term.
Research and investment firm Renaissance Capital said Buhari's government would be "substantially resource-constrained" and his biggest challenge would be managing high expectations.
Buhari won the support of voters largely through his tough stance against corruption. His brief tenure in the 1980s is remembered as a time when bribery and graft were forbidden.
He said his administration would have zero tolerance for corruption but experts warn that maintaining his fragile coalition could involve working with some politicians who have a mixed past.
Buhari is from Nigeria's mainly Muslim north and enjoys massive support in the region.
But he almost certainly would have lost without the backing of partners from the predominately Christian south, including ex-PDP heavyweights who have been linked to graft.
Victims of Boko Haram's brutal insurgency in the northeast have voiced hope that Buhari will do more than Jonathan in tackling the uprising.
Gains have been made since February in an offensive backed by neighbouring Cameroon, Chad and Niger but violence persists and sustained pressure is required to defeat the resilient Islamists.
Containing unrest in the southern, oil-producing Niger Delta may prove to be just as tough a problem.
The delta is Jonathan's home region and some former militants who fought for a fairer share of oil revenues in the 2000s had threatened to resume violence if their native son was defeated.
A 2009 government amnesty programme that saw oil rebels swap guns for regular cash stipends has significantly reduced unrest but the scheme expires this year.
Cancelling the amnesty programme risked "plunging Nigeria's oil-producing region back into an armed insurgency", said analyst Malte Liewerscheidt at the Verisk Maplecroft consultancy group.