Diaz opened the contest with his hands held high, attempting to block Pacquiao's punches but the Filipino soon found a way behind his opponent's guard, landing right hooks from a southpaw stance and also penetrating with sharp uppercuts.

Diaz was bleeding from the bridge of the nose in the second round and by the third, he had blood gushing from a cut above the right eye.

Referee Vic Drakulich stopped the action on several occasions for the ringside physician to confirm that Diaz was able to continue.

"I was kind of surprised the fight wasn't stopped earlier," Pacquiao told reporters. "It was a really bad cut."

Pacquiao repeatedly rattled Diaz with powerful blows and on the rare occasion when the American did land punches of his own, his opponent retaliated with more violent flurries.

ELUSIVE PACQUIAO

As the fight progressed, Diaz launched punches with increasing desperation but frequently missed his elusive opponent.

"It was his speed," Diaz said. "It was all his speed. He boxed much more than I thought he would."

Pacquiao's trainer Freddie Roach was full of praise for his fighter after the bout.

"He boxed better than he ever has," Roach added. "It was beautiful. We told him not to stand and trade in front of this guy, because he's too dangerous.

"Go in and out, do what you do best."

By the ninth round, Diaz appeared weary and his face bloodied as Pacquiao moved in for the kill.

The end came when the new champion snapped back his foe's head with a right jab and followed up with a booming left that dropped Diaz to his knees.

Diaz then collapsed on to his face and rolled onto his back and Drakulich waved off the contest without a count.

The sudden violence of the knockout shocked even Pacquiao, who immediately tried to pull Diaz to his feet.

"My first concern was for Diaz," he said. "I was praying he was alright. I tried to pick him up."

It was Pacquiao's first bout in the lightweight division and the victory added to his titles at flyweight, super bantamweight, and super featherweight.

He weighed 106 pounds on his professional debut in 1995, 29 pounds lighter than he weighed on Saturday, although he said the greater weight suited him and he planned to continue campaigning in the division.

"I feel much stronger and more powerful at this weight," he said.