By Martyn Herman
Katsidis had too much firepower for the previously unbeaten Mitchell, who was roared on by 15,000 fellow east Londoners at the home of soccer club West Ham United.
Mitchell remained composed behind his jab despite the onslaught from his compact 29-year-old rival but was backpedaling throughout and it seemed only a matter of time before the marauding Katsidis caught up with him.
The Londoner was rocked by two big left hooks early in the third and despite connecting with a pair of solid rights of his own he was nailed again by a thudding left from Katsidis midway through the round that wobbled his legs.
Referee Dave Parris jumped in quickly to protect Mitchell from serious damage as Katsidis fired off a blurring succession of unanswered punches.
Katsidis, whose only defeats in 28 professional bouts came at the hands of world class operators Joel Casamayor and Juan Diaz, said the electric atmosphere had inspired him.
"It was fantastic, it was a pleasure to be here," he said. "I feel I'm the best I've ever been, I've been training hard and I've been away from my family for three months.
(Editing by Tony Jimenez and Peter Rutherford)