The New York-based rights group reported that it had confirmed 19 cases of abuse since December 12, "all apparently to press Burundians to vote yes on the referendum" that is scheduled for May 17.
State agents and members of a violent youth group allied with the ruling party, the Imbonerakure, have used repression to ensure the vote goes in the president's favor, and their victims include a man who was beaten to death and another whose beating may have resulted in his death, the group said in its report.
"There is little doubt that the upcoming referendum will be accompanied by more abuses," said Ida Sawyer, Central Africa director at Human Rights Watch. "Burundian officials and the Imbonerakure are carrying out violence with near-total impunity to allow (the president) to entrench his hold on power."
Burundi's government denied the charges it targets perceived opponents, saying Human Rights Watch's report is "full of lies and fabrications."
"Those who have been arrested unjustly, let justice do its job .... There has never been a plan to harass this or that one because of his opinion, because even our constitution forbids it," said Therence Ntahiraja, the assistant minister of home affairs. "Yes, there may be some tensions here and there, but let's avoid associating that with this plan of modifying the constitution."
Burundi has been plagued by violence since April 2015, when deadly protests broke out after President Pierre Nkurunziza sought a third term, leaving an estimated 1,200 people dead. More than 400,000 people have fled the country.
The International Criminal Court judges last year authorized an investigation into allegations of state-sponsored crimes during the recent political upheaval in the East African nation that the U.N. human rights chief has called one of "the most prolific slaughterhouses of humans in recent times."
Nkurunziza rose to power in 2005 following the signing of the Arusha accords, which ended Burundi's civil war that killed about 300,000 people. He was re-elected unopposed in 2010 after the opposition boycotted the vote. He said he was eligible for the third term in 2015 because lawmakers, not the general population, had chosen him for his first term. Critics called the move unconstitutional.
In the upcoming referendum Burundians will say yes or no to a proposal to extend the president's term from five years to seven, which means Nkurunziza could rule for another 14 years when his current term expires in 2020.
Associated Press reporter Eloge Willy Kaneza in Bujumbura, Burundi, contributed to this report.
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