Burundi's ruling party is waging a "relentless campaign of intimidation" against its critics ahead of presidential elections next year, Amnesty International says.
Its youth wing was responsible for attacking and killing opposition supporters, the rights group said.
The CNDD-FDD party has not has yet commented on the allegation.
Burundi is recovering from a brutal civil war that ended with President Pierre Nkurunziza's election in 2005.
Speculation is rife that he intends to run for a third term in the June 2015 election, despite opposition claims that this would be in violation of the constitution which imposes a two-term limit.
The CNDD-FDD's youth wing, Imbonerakure, has "strong links to the security services and are responsible for perpetrating human rights abuses with impunity", Amnesty said in a report.
At least one opposition member was shot and killed by two soldiers on the orders of a local official and an Imbonerakure member, Amnesty said.
"Another opposition member had his teeth knocked out during a beating," the report said.