HRW says in a statement it has collected evidence that at least eight detainees were subjected to ill-treatment that, in some cases, "may amount to torture."
HRW singles out the case of Rasul Kudaev, a former Guantanamo Bay inmate, who was arrested after the 13 October attack. HRW quotes Kudaev's lawyer as saying her client was severely beaten in detention and could not walk when she saw him three weeks ago.
Russian prosecutors have admitted to arresting some 50 people for their alleged participation in the attack. But rights campaigners in Kabardino-Balkariya say the number of detainees is much higher.
Officially, the Nalchik attack claimed nearly 140 lives, including those of 12 civilians.
A slideshow look at the October 13-14 violence in Nalchik, capital of the Russian North Caucasus Republic of Kabardino-Balkaria.