At least 450 people have been killed in violence during protests against Nkurunziza's re-election bid in April last year.
The head of one of Burundi's opposition parties has been released, days after he was arrested for what police said was collaborating with "armed gangs", a spokesman for an opposition coalition said on Tuesday.
The central African state has been in a political crisis and sporadic violence for more than a year, sparked by President Pierre Nkurunziza's decision to run for a third term in office, which he secured in a disputed election in July 2015.
At least 450 people have been killed in violence that first erupted during protests against Nkurunziza's re-election bid in April last year.
Gervais Niyongabo, chairman of the opposition FEDES-SANGIRA party and one of the few opposition leaders still able to work inside Burundi, was detained in the southern Makamba region on Sept. 28, police said.
FEDES-SANGIRA party was among the opposition groups to boycott last year's elections.
"It was but a police fabrication to arrest him, given that his native village is known as an opposition stronghold," Pancrace Cimpaye, spokesman for the opposition grouping CNARED, said.
Police and government officials could not immediately be reached for comment.
Opponents accuse Nkurunziza of violating the constitution and a peace pact that ended a civil war in 2005. The government accuses opponents of stirring unrest, and accuses them of backing rebel groups, which officials call "armed gangs".
On Tuesday, Burundi's government said it had dismissed a U.N. decision to set up a commission of inquiry to identify perpetrators of killings and torture, saying it was based on a one-sided account of events in the country.