Report from the BBC
Former Burundi rebel leader Pierre Nkurunziza has been sworn in as president, marking the end of 12 years of war which has left 300,000 dead.
He becomes the first leader chosen through democratic means since 1993.
It marks the end of a five-year peace process designed to end the conflict between Hutu rebels and an army led by the Tutsi minority.
Power will be shared under the peace deal with Tutsis guaranteed a share of power and government jobs.
"I pledge to fight all ideology and acts of genocide and exclusion, to promote and defend the individual and collective rights and freedoms of persons and of the citizen," he said in the Kirundi language in a ceremony attended by several African heads of state.
Outgoing President Domitien Ndayizeye said this was "the most important day in Burundi's history."
His Frodebu party was defeated in local and parliamentary elections earlier this year by Mr Nkurunziza's Forces for the Defence of Democracy, before MPs elected the ex-rebel as president last week.
"We have won the battle," said Mr Ndayizeye (pictured above).
The BBC's Rob Walker in the capital, Bujumbura, says Mr Nkurunziza's journey to power has been an extraordinary one - from school teacher to rebel leader and now finally, to president.
He has said his first task will be to try and engage the last remaining rebel group, the National Liberation Forces (FNL) in peace talks.
"I hope he will bring back peace quickly and help us overcome poverty," said Fatuma Siniremera, a 56-year-old Nkurunziza supporter during a rally on Thursday.
But some Tutsis remain fearful of Hutu rule.
"I am very pessimistic about whether he will change anything," Dieudonne Hakizimana said.
The power-sharing deal agreed and now finally implemented is seen as a crucial success for the continent and one which could have wider benefits for the volatile Great Lakes region.
Our correspondent says Mr Nkurunziza takes control of a country which is virtually destroyed but which has huge expectations of his ruling party.
He says the new leader will need all the support he can get from the international community if he is now to deliver on the much-needed dividends of peace.
But the challenge ahead is not just physical.
Deep divisions from the civil war remain and many believe those will only be healed if the new government deals with the issue of justice for crimes committed by all sides.
On the eve of his inauguration, six mainly Tutsi parties said Mr Nkurunziza should be brought to justice for crimes they say he committed as a guerrilla leader during the civil war.
A Burundian court passed the death sentence on Mr Nkurunziza in 1998, but he was granted an amnesty in the peace accords.
No elected government has ever served out its term in Burundi.