Marvelous Marvin Hagler knocked down Tony Sibson twice and ripped his face into a bloody mess, stopping the Englishman 2:40 into the sixth round tonight to retain the undisputed middleweight championship of the world.

In the fourth round, Hagler bloodied Sibson's nose with a lead right hand. He then sent blood streaming down the left side of Sibson's face with a three-punch combination in the fifth.

Hagler sensed he was close to a knockout as he came out for the sixth round and he let go three straight right hands to Sibson's eye, which reopened the cut immediately.

Another right-left combination sent Sibson backward and Hagler switched from left-handed to right-handed. He connected with a right-left combination to the head and Sibson toppled backward on the seat of his pants. He popped up at the count of two and tried to counterattack, but Hagler was too much.

Hagler managed four quick punches to the head and a left to the body which doubled over Sibson. Two chopping right hands to the top of Sibson's head sent him face down to the canvas.

He struggled to his feet and took out his mouthpiece, but referee Carlos Padilla took one look at him and stopped it.

The bout was fought before an overflow crowd of nearly 14,000.

Many boxing observers had considered Sibson, 24, to be the toughest challenger to Hagler, 28, since he won the title by stopping Allen Minter Sept. 27, 1980. Sibson had held the European middleweight championship until he relinquished it recently and he was ranked the No. 1 contender by the WBC.

Hagler was not impressed. "All they are doing is sending this kid to a moment of destruction," he had said.

With the victory, Hagler raised his record to 56-2-2 with his 47th knockout. He has not lost in his last 31 fights, going 30-0-1.

Sibson fell to 47-4-1 and suffered his first loss in his last 15 fights.