0801 GMT April 22, 2018
Fighters beat women, burned them and left them with broken bones, smashed teeth and head wounds, the New York-based rights group said in a report, citing testimony, aljazeera.com wrote.
Survivors said they were raped by up to 10 or more men during a single incident.
According to the report, there have been 305 cases of rape and sexual slavery against 296 women and girls by members of armed groups between early 2013 and mid-2017.
"Armed groups are using rape in a brutal, calculated way to punish and terrorize women and girls," said Hillary Margolis, HRW's women's rights researcher, in a statement.
She added: "Every day, survivors live with the devastating aftermath of rape, and the knowledge that their attackers are walking free, perhaps holding positions of power, and to date facing no consequences whatsoever."
According to HRW, most of the abuses not only violate CAR's laws but also constitute war crimes, with some possible crimes against humanity.
The group reported that only 11 of the 296 survivors interviewed had tried to initiate a criminal investigation.
So far, no member of any armed group has been arrested or tried for committing sexual violence.
A new government organization, the Mixed Unit for Rapid Intervention and Suppression of Sexual Violence against Women and Children, is trying to address the plight of survivors.
"If those that we know are responsible for these grave crimes — of rape and violation of human rights — if they are not arrested right now and taken into custody, no victims will have the will to seek justice and that makes justice inaccessible," said Paul Amedee Moye, head of the group.
He added: "You have to talk about zero impunity."
CAR, a former French colony, fell into a protracted political crisis in 2013 after President Francois Bozize was overthrown.