Mr Pierre Buyoya, who has led Burundi since seizing power in a 1996 coup, was sworn in to head a new power-sharing government aimed at ending eight years of ethnic war between Tutsis and Hutus.
Mr Buyoya becomes president for the first 18 months of the three-year transition regime, which will establish a more ethnically balanced cabinet, parliament and army.
A leading Hutu opposition figure, Mr Domitien Ndayizeye, was also sworn in as vice president. He will become president during the second half of the administration.
The post of vice president will then go to a Tutsi, although Mr Buyoya has made it clear he will not take up this post.
Twenty-six ministers appointed by decree on Tuesday were at today's ceremony in Bujumbura, along with former South African president Mr Nelson Mandela, who brokered two years of peace negotiations here.
Some 250,000 people, mostly civilians, have died in Burundi since Hutu rebels launched an insurgency in 1993.
AFP