Government-allied youth in Burundi have gang-raped women and girls whose male relatives are suspected to be opposition activists, campaign group Human Rights Watch (HRW) has said.
Some of them were tied up and raped at gun or knife point, as their families and children watched, it added.
HRW blamed most of the rapes on Imbonerakure, the youth wing of the ruling party, and the security forces.
Burundi's government described the allegations as "falsehoods".
Imbonerakure played a key role in helping President Pierre Nkurunziza remain in power following his decision in April 2015 to seek re-election.
He survived a coup attempt and mass protests and won subsequent elections.
At least 439 people were killed and 240,000 fled to neighbouring states during the unrest, according to the UN.
HRW said that 323 cases of rape or sexual assault, affecting 264 women and 59 girls, were reported from May to September 2015.
It has interviewed more than 70 survivors who have fled to a refugee camp in Tanzania.
"In a pattern of abuse in many locations and in several provinces, men armed with guns, sticks, or knives have raped women during attacks on their homes, most often at night," HRW said in a report.
"Many of the women have suffered long-term physical and psychological consequences," it added.
HRW quoted a 36-year-old survivor as saying: "I was held by the arms and legs. [An attacker] said: 'Let's kill her, she is an [opposition National Liberation Forces] FNL wife' as they raped me."
Other rape cases documented by HRW included:
- A mother finding her eight-year-old daughter "sitting in bloody sheets" after being raped by four youths
- A 17-year-old being dragged to a banana grove near the family home and gang-raped
- A 22-year-old being beaten with sticks and raped by two youths who forced their way into the family home
- A woman being picked up from her bed, thrown to the ground and raped by two youths.
"Women said that if the man wanted by the attackers was not there, they would demand to know his whereabouts and would sometimes tell the victim that they were raping her because they could not find the man," HRW said.
"In nine cases, women said the men had fled before the rape took place, or had begun habitually sleeping elsewhere because of threats," it added.
Presidential spokesman Willy Nyamitwe said HRW had damaged its credibility through the "publication of falsehoods".
"Imbonerakure is not a gang of rapists," he added.
Timeline - Burundi crisis
- April 2015 - Protests erupt after President Pierre Nkurunziza announces he will seek a third term in office.
- May 2015 - Constitutional court rules in favour of Mr Nkurunziza, amid reports of judges being intimidated. Tens of thousands flee violence amid protests.
- May 2015 - Army officers launch a coup attempt, which fails.
- July 2015 - Elections are held, with Mr Nkurunziza re-elected. The polls are disputed, with opposition leader Agathon Rwasa describing them as "a joke".
- November 2015 - Burundi government gives those opposing President Nkurunziza's third term five days to surrender their weapons ahead of a promised crackdown.
- November 2015 - UN warns it is less equipped to deal with violence in Burundi than it was for the Rwandan genocide.
- December 2015 - 87 people killed on one day as soldiers respond to an attack on military sites in Bujumbura