AMNESTY International says an independent and transparent investigation should be carried out into the assault of two men as shown on video footage.
The organisation said torture was unacceptable under any circumstances and those responsible should be brought to justice.
In a statement, Amnesty International said video footage apparently showing the torture of two men was shocking.
"The nine minute video posted online appears to show two men being repeatedly beaten with poles as they lie huddled on the ground handcuffed and screaming in agony as batons are used repeatedly against them," said Amnesty International.
It said the authenticity of the footage was still to be verified and the perpetrators were not in uniform.
"The Fijian authorities must treat this shocking footage with the utmost seriousness and immediately initiate an independent investigation," said Roseann Rife of Amnesty International.
"While the video is still to be verified, what is clear is that torture is unacceptable under any circumstances and those responsible must be brought to justice.
"The humiliation of the men and their injuries evident in the video is very serious.
"Forced to undress and harassed by a dog as men nearby laugh, it is difficult to watch.
"The subsequent brutal beating with batons is harrowing. It is torture," said Ms Rife.