Nigerian election winner Muhammadu Buhari has said his government would "spare no effort" to defeat the Islamist militant group Boko Haram.
"Boko Haram will soon know the strength of our collective will. We should spare no effort," he said in his first formal speech since winning the election.
"In tackling the insurgency, we have a tough and urgent job to do."
Mr Buhari congratulated outgoing president Goodluck Jonathan for peacefully relinquishing power, a day after becoming the first politician in Nigeria's history to remove a sitting leader at the ballot box.
In an unprecedented step, Mr Jonathan phoned Mr Buhari to concede defeat and urged his supporters to accept the result, a signal of deepening democracy few had expected.
"President Jonathan was a worthy opponent and I extend the hand of fellowship to him," Mr Buhari, wearing a black cap and kaftan, told reporters and supporters to loud applause.
"We have proven to the world that we are people who have embraced democracy. We have put one-party state behind us."
The 72-year-old general, who first came to power three decades ago via a military coup, campaigned as a born-again democrat intent on cleaning up Nigeria's corrupt politics.
The margin of victory – Mr Buhari took 15.4 million votes to Mr Jonathan's 13.3m - was enough to prevent any challenge.
"President Jonathan has placed his country's interests first by conceding the election," US President Barack Obama said.
"The context has changed ... There have been 16 years of democracy, there's a constitution, there are legal safeguards," British High Commissioner Andrew Pocock told BBC radio.
Investors are also cautiously optimistic that any crackdown on corruption by Mr Buhari will stimulate investment and boost flagging growth in Africa's biggest economy.
The rules state that Jonathan must officially hand over on 29 May.
His People's Democratic Party has been in charge since the end of army rule in 1999, but had been losing support due to several oil sector corruption scandals and the government's performance in combating Islamist Boko Haram militants.
"You voted for change and now change has come," Mr Buhari said.
Cities in largely Muslim northern Nigeria, where Mr Buhari's core support base lies, erupted in celebration.
Mr Jonathan's appeal to his supporters that "nobody's political ambition is worth blood" meanwhile helped calm their frustrations, reducing the chance of post-election violence that blighted the 2011 poll, when the result was reversed.
Despite the killing of more than a dozen voters by Boko Haram gunmen - who had pledged to derail the poll - the election was one of the most orderly in the country's history.
Mr Buhari took power in a 1983 coup only to be thrown out 18 months later by another general.
He subsequently embraced democracy, running in several elections and despite losing always bounced back.
He acknowledged the hard work ahead in building bridges.
