Owen Haggerty, 22, was just a day away from finishing his first deployment on the Middle Eastern island when he tried to stop a street brawl last month.
He was dragged away by police and was jailed for three months without being allowed to defend himself in court.
His shattered older brother Jack, 29, flew out to support him four weeks ago but was left in disbelief as terrified Owen, from Johnstone, Renfrewshire, was banged-up.
Shocked Jack said: "As he walked away I was completely helpless because I don't know what the next step is.
"That was the last time I got to see him. The situation is just horrendous."
Owen had spent four months serving on minesweeper HMS Bangor when a row broke out on a night out with friends off base to mark the end of his deployment.
He told police he had stayed to help the victim and says he saw CCTV that night clearly showing he was innocent in the altercation.
But Owen was held in a detention centre until he was locked up.
Jack - who flew home to Glasgow yesterday to continue the fight to get him released - said: "There is not a single bit of evidence other than Owen physically staying with the gentleman and helping him.
"It would have been...