The United States earlier this month denounced “violence, intimidation and harassment” against those thought to oppose the referendum, and expressed concern about the “nontransparent process” of changing the Constitution.
Human Rights Watch has noted “widespread impunity” for the authorities and their allies, including the ruling party’s youth wing, as they try to swing the vote in the president’s favor.
Many in Burundi, a poor country that still relies heavily on foreign aid, worry that a new round of bloodshed will follow the referendum whatever its results.
Already, more than 400,000 people have fled the country since the political unrest began in April 2015, according to the United Nations.
Mr. Nkurunziza, a former rebel leader, rose to power in 2005 after the end of a civil war that killed about 300,000 people. He was re-elected unopposed in 2010 after the opposition boycotted. He said he was eligible for a third term in 2015 because lawmakers, not the general population, had chosen him for his first term.