Commissioner for Human Rights to conduct an independent international investigation.
"The government's response to allegations of sexual violence by its security forces have been dismissive, deeming them as 'fake' or 'pro-LTTE propaganda'.
"It's not clear who in the government knew about these horrific crimes. But the government's failure to take action against these ongoing abuses is further evidence of the need
for an international investigation," Brad Adams, Asia director at HRW, said in a statement.
"The Sri Lankan security forces have committed untold numbers of rapes of Tamil men and women in custody. These are not just wartime atrocities but continue to the present, putting every Tamil man and woman arrested for suspected LTTE involvement at serious risk," he added.
HRW said most of the Tamils it interviewed for its report spoke with its investigators outside of Sri Lanka and researchers corroborated their stories with medical and legal records.
"Because Human Rights Watch was unable to openly conduct research in Sri Lanka or interview people still in custody, these cases likely represent only a tiny fraction of custodial rape in political cases.
"Many of the cases followed a pattern of an individual being abducted from home by unidentified men, taken to a detention centre, and abusively interrogated about alleged LTTE activities," the report said.
Sri Lanka has always denied allegations that it targeted civilians during the war and resisted calls for an independent inquiry, establishing its own Lessons Learned and Reconciliation Commission.