Electoral agency says 73pc voted "Yes" in the referendum.
The turnout was 96 per cent.
The results must be validated by the constitutional court within nine days.
Burundians have overwhelmingly voted for constitutional reforms bolstering President Pierre Nkurunziza's powers and giving him the option to stay in power until 2034, official results showed Monday.
Election commission chief Pierre-Claver Ndayicariye said 73 per cent of voters had voted "Yes" in a referendum to change the constitution, and 19 per cent had voted "No."
The turnout was 96 per cent, he said.
The results — which exclude figures from the diaspora representing 0.27 per cent of the vote — are provisional and must be validated by the constitutional court within nine days.
Observers had widely expected the reforms to pass, partly due to support Nkurunziza still holds in rural areas, but also due to a three-year crackdown on dissent.
Nkurunziza, 54, who has been in power since 2005, plunged his tiny East African nation into crisis in 2015 when he sidestepped a constitutional two-term limit, arguing his first term came after an election by parliament.
The move sparked angry protests, a government crackdown, coup attempt and widespread abuses which prompted the International Criminal Court to launch a probe into the atrocities.
This angered Burundi, which became the first country to withdraw from the ICC.
At least 1,200 people have died and 400,000 been displaced, according to the ICC.
The constitutional reforms, which include measures that hand more power to Nkurunziza and his ruling CNDD-FDD, change term limits to seven years, meaning he could start again from scratch in 2020.
Critics say the referendum has struck a death blow to the Arusha peace deal, signed in 2000.
The accord ended Burundi's 1993-2006 civil war and ushered in measures to ensure power would not be concentrated in either the hands of the majority Hutu or minority Tutsi, after decades of violence between the communities.
