Amnesty says ruling party's youth wing has "strong links to the security services and are responsible for perpetrating human rights abuses with impunity".
Burundi's ruling party is carrying out a "relentless campaign of intimidation" against opposition and critics, ahead of presidential elections next year, Amnesty International said Tuesday.
"The government's clampdown on free expression and peaceful assembly has serious implications for human rights ahead of next year's elections," said Amnesty researcher Tom Gibson.
Burundi emerged in 2006 from 13 years of brutal civil war and its political climate remains fractious ahead of presidential polls due in June 2015. President Pierre Nkurunziza, in power since 2005, is believed to be planning a third term in office, which his opponents claim would violate Burundi's constitution.
The presidential vote is scheduled for June 2015.
The rights groups warned of a "crackdown on freedom of expression, association and peaceful assembly," as well as a "sharp increase in politicised violence" linked to the ruling CNDD-FDD party.
Its youth wing, know as the Imbonerakure, has "strong links to the security services and are responsible for perpetrating human rights abuses with impunity," Gibson added, accused the group of "intimidation, harassment and violence, attacking and even killing members of the political opposition."
One UN official has said there are an estimated 20,000 Imbonerakure members.
"Opposition groups and civil society have been arbitrarily denied permission by the authorities to meet publicly," Amnesty said, adding that a press law and draft law on non-profit organisations "pose a further threat to free expression and peaceful assembly."
A United Nations official was expelled in April after a confidential note reporting the distribution of weapons by the government to the Imbonerakure was leaked.