Amnesty International says the investigation into a graphic video, apparently depicting Fiji police or military officers beating detainees must be transparent.
Fiji police say investigations are underway into the video, posted online.
The nine-minute recording shows the faces of several of the men involved in the abuse and also shows the licence plate of a pick-up truck, on which one of the tied-up men was beaten and stripped.
The Head of Amnesty International's East Asia Division, Roseann Rife, is calling on other governments to play in role in pushing for an open investigation.
"We are glad to hear there is an investigation going on but we are calling on them to make sure it is in fact an independent investigation but very transparent that's the only way it's going to gain the confidence of the Fijian people that it was a real investigation into this horrible act"
The Head of Amnesty International's East Asia Division, Roseann Rife.
In a statement, the police say they understand the two victims are recaptured prisoners.