Ashita no Joe: Fighting for Tomorrow(1968)

Ashita no Joe

Joe Yabuki is a troubled young man who runs away from an orphanage. Wandering through the Tokyo slums, he meets former boxing trainer Danpei. Joe is later arrested and goes to a temporary jail where he fights Nishi, his future best friend, ​and leader of a group of hooligans. He and Nishi then go to a juvenile prison miles away from Tokyo. There Joe meets Rikiishi, a former boxing prodigy, and a rivalry develops between them. They face each other in a match in which Rikiishi dominates Joe until the latter hits him with a cross-counter, resulting in both being knocked out. This inspires the other prison inmates to take up boxing. Joe and Rikiishi vow to fight again.

(Source: Wikipedia)