Burundi's newly elected president Evariste Ndayishimiye was sworn in Thursday, after the sudden death of his predecessor who left him an isolated nation in political and economic turmoil.

Ndayishimiye was elected in May in a vote disputed by the opposition, and was meant to take office in August, however the inauguration was sped up after his predecessor Pierre Nkurunziza's shock death.

Nkurunziza, who ruled the east Africa nation for 15 often tumultuous years, was said by the government to have died of a heart attack last week.

Nkurunziza, a devout evangelical who believed he was chosen by God to lead Burundi, leaves a "dark and sad legacy", Carina Tertsakian of the Burundi Human Rights Initiative said.

After the news of Nkurunziza's death Ndayishimiye vowed to "continue his high-quality work that he has done for our country".