Former Burundian MP Fabien Banciryanino is in police detention for what the authorities said was an attempt to overthrow the government, according to his family.
Mr Banciryanino was arrested on Friday after a press conference that he had called for that day was cancelled, a close family member told the BBC.
He was taken by police from his home in the commercial capital, Bujumbura, the family member added.
Police have not commented on the claims.
The former lawmaker was a critic of ex-President Pierre Nkurunziza, who died in June. He opposed a law that created a number of retirement perks for Mr Nkurunziza, including giving him the title of “supreme guide to patriotism”.
His press conference on Friday was to comment on accusations carried by a local YouTube channel that he had insulted the new President, Evariste Ndayishimiye, according to the family member.