The winner of Nigeria's presidential election, Muhammadu Buhari, has issued a defiant vow against militants Boko Haram, saying they would "soon know the strength of our collective will".
The president-elect also vowed to tackle corruption, which he described as an "even greater evil".
Gen Buhari won the election beating President Goodluck Jonathan by 2.5 million votes.
World leaders congratulated Gen Buhari and hailed Nigeria's democratic spirit.
Gen Buhari, of the All Progressives Congress (APC), has become the first opposition candidate to win a presidential election in Nigeria.
In a televised speech, he said: "In tackling the insurgency, we have a tough and urgent job to do. Boko Haram will soon know the strength of our collective will. We should spare no effort until we defeat terrorism."
Much of Gen Buhari's support was in the north, in particular the north-east, which has suffered from Boko Haram's six-year insurgency. The militants have killed thousands of people in their drive to establish an Islamic state.
Many people accused Mr Jonathan of not taking Boko Haram seriously and thought Gen Buhari would be better positioned to defeat the militants.
Analysis: BBC's Will Ross in Lagos
This is a hugely significant moment in Nigeria's turbulent history. Never before has a sitting president been defeated in an election.
Since independence from Britain in 1960, there have been numerous coups and although the 2011 vote was an improvement, most elections have been rigged or even annulled by the military.
Of course in a relatively close election, there will be millions of people who are not pleased with the outcome. But the whole process is a sign that democracy is deepening in Nigeria and may be a tonic to other countries in Africa.
Nigerians can start to believe that it is possible to remove politicians through the ballot box.
Gen Buhari, 72, also angrily denounced corruption, saying: "It creates unjustly enriched people... and undermines democracy. Corruption will not be tolerated by this government."
He praised his defeated rival, saying: "I extend a hand of friendship and conciliation to President Jonathan and his team. I have no ill will against anyone. He has nothing to fear from me. He is a great Nigerian and still our president."
Pledging to uphold the rule of law, the former military ruler said: "Your constitutional rights remain in safe hands - you will be able to voice your opinion without fear of victimisation."
Gen Buhari pledged a government "that will listen to, and embrace, all", adding that "there shall be no bias or favouritism for any Nigerians".
He said: "Our nation faces many challenges - insecurity, corruption, economic decline. I pledge to give you my best in tackling these problems."
Earlier, US President Barack Obama said the polls had "shown the world the strength" of Nigeria's commitment to democracy.
He praised both Gen Buhari and Mr Jonathan for "their public commitments to non-violence throughout the campaign".
Muhammadu Buhari in focus:
- Muslim from northern Nigeria, aged 72
- Military ruler of Nigeria from 1984 to 1985, deposed in a coup
- Seen as incorruptible
- Poor human rights record
- Disciplinarian - civil servants late for work had to do frog jumps
- Survived apparent Boko Haram assassination attempt
UN Secretary General Ban Ki-moon commended Mr Jonathan for "his statesmanship in upholding the democratic process" and said "the successful conduct of these polls [was] a testament to the maturity of Nigeria's democracy".
UK Foreign Secretary Philip Hammond said: "It is now important for both parties to ensure the transition to a new government remains peaceful."
More than 28 million Nigerians turned out to vote in the elections over the weekend. Gen Buhari beat Mr Jonathan by 15.4 million votes to 12.9 million.
Mr Jonathan said in an earlier statement: "I promised the country free and fair elections. I have kept my word."
He called on his supporters to stay calm, saying: "Nobody's ambition is worth the blood of any Nigerian."
Mr Jonathan's spokesman Reuben Abati told the BBC the president would remain in office until the handover date of 29 May.