The comments by Melchiade Nzopfabarushe, made during a rally on Sunday to party members in his native village, came amid increased political jitters in Burundi, whose modern history has been marred by ethnically-charged civil war.
Nearly 430,000 people, including opposition politicians, have fled the tiny East African nation of 10.5 million since Nkurunziza won a third term in a 2015 election that sparked violent clashes. His foes said he had no right to run again.
“We said that we have ordered boats. We will send them (opponents) into Lake Tanganyika,” Nzopfabarushe said in a video clip of his comments, which have been circulating online, to the party members in Kabezi, near the capital Bujumbura.
“He who has the president’s support successfully achieves his endeavours. That is the message we are giving either here or nationwide,” said Nzopfabarushehe, a former senior official in the president’s office.
The judicial source told Reuters that Nzopfabarushe was being held in custody in the capital.
Though he did not confirm the arrest, Justice Ministry spokesman Adolphe Havyarimana told reporters that Nzopfabarushe would appear in court on Monday.
The ruling CNDD FDD party sought to dissociate itself from Nzopfabarushe’s comments, saying on Twitter it rejected “any subversive message which may jeopardise unity and cohesion among the Burundian people”.
The party also said it had asked the justice ministry to investigate all cases of divisive language used by politicians ahead of the May 17 vote.
The referendum would extend the presidential term to seven years from five, allowing Nkurunziza to run again in 2020. It would limit the president to two consecutive seven-year terms, but would not take into account previous terms, potentially extending his rule to 2034.
Nkurunziza came to power in 2005 after a peace deal ended a decade of civil war between the Tutsi-dominated army and Hutu rebels, in which 300,000 people were killed.
He ran for a third term in 2015, which opponents said violated the terms of the peace deal, sparking clashes that resulted in hundreds of deaths.
Human rights groups say the referendum will not take place in a free and fair climate.