Eight years ago, Kewell was a member of the United side beaten 2-0 by Galatasaray in a UEFA Cup semi-final first leg clash in Istanbul, which went ahead just 24 hours after two Leeds supporters, Kevin Speight and Christopher Loftus, were murdered on the streets of the Turkish capital by hooligan Galatasaray followers.
Ray Fell, chairman of Leeds United Supporters Club, told the News of the World: "Many of us are appalled by Kewell's decision.
"They will see this as callous, uncaring and a real kick in the teeth to them, the two lads who so tragically lost their lives and the football club.
"I am shocked and extremely disappointed that a member of that Leeds squad, who along with his team-mates, the families of Kevin and Christopher and everyone else connected with the club went through the trauma and sadness of that occasion, has signed for Galatasaray.
"For us now to have to see one of our ex-players from that sad time taking the applause from Galatasaray fans will be sickening."
Kewell spent eight years at Elland Road before leaving for Merseyside. He joined the Reds at a time when Leeds were having to off-load their top stars because the club had hit financial difficulties.
Fell added: "Nobody here had a bad word to say about Harry when he went to Liverpool because the fans could understand the reasons why he was leaving.
"There was no blame attached to him then, but this is different. The fans won't forgive him for this."