HRW investigations found the youths "used clubs to beat to death a 15-year-old, drove a knife into the eye of one victim ... and attacked others".
Young men belonging to Burundi's ruling party are growing increasingly powerful, often collaborating with national intelligence agents as they wage brutal attacks on perceived opponents, Human Rights Watch said Thursday.
The US-based rights group said the youths, known as Imbonerakure -- "those who see far" in the local Kirundi language -- had been involved in arresting and attacking opposition members since the start of a political crisis in the country in April 2015.
In the past three months HRW investigations found the youths "used clubs to beat to death a 15-year-old, drove a knife into the eye of one victim ... and attacked others with knives, clubs and wooden poles.
"Imbonerakure members have also set up unofficial roadblocks in multiple provinces, sometimes detaining and beating passersby and extorting money or stealing their possessions," read a statement.
Burundi was plunged into crisis when President Pierre Nkurunziza launched a bid for a disputed third term in office, prompting massive protests and a failed coup attempt.
Ensuing unrest has left more than 500 people dead and seen more than 300,000 flee the central African country.
HRW cited witnesses saying the Imbonerakure sometimes worked side-by-side with police or the military, arresting perceived opponents and taking them to detention centres to be tortured or beaten.
The rights group interviewed over 20 victims and human rights activists since October.
One man from a northern province described "a hunt for the opposition" in which he and another person were tied up and taken to the police station."
"They told us: 'We are going to correct you'," he said.
The man witnessed one of the youths stabbing a fellow detainee in the eye saying "next time you will see far."
According to the HRW probe, while some Imbonerakure are arrested for committing abuses, many are quickly released and never face trial.
In a response to questions from HRW, Burundi communication official Nancy-Ninette Mutoni said the ruling party had received no complaints of abuses.
However, she said the Imbonerakure "not only have the right but also the obligation to do surveillance and to signal all movements and suspect acts to the security forces."
Nkurunziza, 53, has worked to shore up control of the country with a crackdown on opponents, the media and civil society.
Burundi has also moved to quit the International Criminal Court which was investigating the country, and cut ties with the UN's main human rights body after a damning September report detailed atrocities and warned of "genocide".
The European Union and United States have slapped sanctions on a handful of high-ranking individuals in Burundi's government, and the EU has also suspended aid to the country.
HRW called for more targeted sanctions, and for the international community to probe abuses by the Imbonerakure.