Explosions and gunfire were heard late Monday in Burundi's capital Bujumbura, hours before polls were due to open in a divisive presidential election, in which Pierre Nkurunziza's is seeking a controversial third term.

AFP journalists heard at least three loud explosions and sustained bursts of gunfire, without being able to determine their origin.

Witnesses said unknown assailants in Ngagara district had opened fire on police officers who fired back. Explosions and shots were also heard in Nyakabiga and Kanyosha districts, residents said.