In the 9-minute video, two men are repeatedly beaten with poles as they lie huddled on the ground handcuffed and screaming in agony, the human rights organization said.
"This appalling incident appears to be the latest example of abuse by the military," said Roseann Rife of Amnesty International. "The Fijian authorities must treat this shocking footage with the utmost seriousness and immediately initiate an independent investigation."
The authenticity of the video has yet to be verified but it appears consistent with earlier reports of brutality against prisoners by the Fijian military, Rife said.
She said before the beating, the two prisoners were forced to undress and were harassed by a dog.