Hopkins, 34, completed his ninth title defence when the referee stepped in to stop a sustained assault. Hopkins, wearing trunks once worn by the late champion Sugar Ray Robinson, dropped Allen in the sixth and stunned him in the fifth.
After the fight Hopkins complained about his purse and claimed that boxers are "being mistreated and exploited as whores". He said the sport was trying to blackball him because of his stance against promoters, whom he assailed in recent hearings in New York on the business of professional boxing.
"They tried to blackball me from the world of boxing, but they can't do it," Hopkins said. "I don't like a lot of stuff that goes on in this business. I'm standing up for my rights. Fighters are taken advantage of. It was a disgrace I was only paid $225,000 (pounds 134,000) for this fight."
The fight itself, a rematch of a controversial no contest bout in August, was an untidy affair. Hopkins hurt Allen with two low blows in the fourth round, then dominated the rest of the way.
Low punches also played a part in the night's other fight. The American Sharmba Mitchell, who lay on the canvas in agony after a low blow in the seventh, recovered to retain his World Boxing Association super-lightweight title with a unanimous decision over Pedro Saiz from the Dominican Republic.Reuse content