Polling stations opened at 6 a.m. local time (4 a.m. GMT), and were expected to close at 4 p.m.
These presidential, legislative and municipal elections in this country located in the Great Lakes Region follow those of 2015, won by the ruling National Council for the Defence of Democracy-Forces for the Defence of Democracy (CNDD-FDD).
Six candidates are seeking votes from the 5,126,351 registered voters, according to the National Independent Electoral Commission (CENI), headed by Pierre Claver Kazihise.
Incumbent president Pierre Nkurunziza is not taking part,
His CNDD-FDD chose General Evariste Ndayishimiye, who is considered to be the favourite to succeed Mr. Nkurunziza.
Ndayishimiye faces the main opposition candidate, Agathon Rwasa of the National Council for Liberty (CNL), who is drawing crowds in the capital, Bujumbura, and in provincial areas.