Nearly 5 million registered Burundians are voting in a controversial referendum that could allow the country's current president to stay in power until 2034.
Long lines were seen at the polling stations early Thursday for a referendum that has been denounced by the opposition.
"Before going to my other activities, I preferred to first perform this civil and patriotic duty," Thacien Kabura, a voter in the capital Bujumbura told Anadolu Agency.
Should Burundians vote majoritarily yes, the presidential terms would be extended from five to seven years, which would allow current President Pierre Nkurunziza to remain in power for two more terms after the current one ends in 2020.
The opposition has criticized the move saying the amendments aim to grant a presidency for life to Pierre Nkurunziza, who has been in power since 2005.
Soldiers and police officers have been deployed in front of each polling station as authorities, as well as observers, fear potential violence.
UN Human Rights Chief Zeid Ra’ad al Hussein on Tuesday expressed concern about a possible upsurge in violence in Burundi.
Last Friday 26 people, among them 10 women and 11 children, were killed in an attack launched by unidentified armed men near the capital Bujumbura. Whether this attack was politically motivated has not yet been determined.
Unrest in Burundi started in April 2015 when Nkurunziza announced his candidacy for a controversial third term whereas only two terms were allowed by the constitution.
The crisis had led to hundreds of deaths and forced hundreds of thousands to flee the country and seek refuge in neighboring states, according to the UN.