During his swearing in on Thursday, Nkurunziza warned rebels that they would be crushed by God.
He thanked God for his win in the elections last month — polls the United Nations say were not free or fair — after taking the oath of office in a surprise ceremony in the capital Bujumbura announced only hours before.
“The victory we have achieved is a victory of all Burundians, those who elected us, and those who did not,” Nkurunziza said.
Nkurunziza’s third term has been condemned as unconstitutional by the opposition and provoked months of protests.
There has been a string of killings since his re-election, including that of his top security chief, assassinated in a rocket attack last month.
But Nkurunziza, an ex-rebel turned born-again Christian who believes he is in power by divine choice, warned those who have chosen “the path that leads nowhere, who attack and fight their country” will be stopped by the hand of God.
“They will be scattered like flour thrown into the air — as the God of heaven is a witness, the Burundians will be at peace,” he said.
No foreign head of state was present, but several African countries, as well as China and Russia, sent their ambassadors. European Union nations and the United States sent lower-ranking officials.
In his oath, Nkurunziza swore loyalty to the constitution and “to dedicate all my forces to the defence of the best interests of the nation, to assure national unity and the cohesion of the Burundian people, social peace and justice.”
But Nkurunziza also said he would review possible changes to a key clause in the constitution — which requires ministers to come from a party with at least five percent of the national vote — to allow the formation of a unity government.
Amid opposition boycotts, some lawmakers are technically on independent lists, making them ineligible to join the government.