Juan Marquez KO’s Manny Pacquiao in the sixth.

LAS VEGAS — Juan Manuel Marquez knocked Manny Pacquiao out cold with a vicious right hand at the end of the sixth round Saturday night, putting a ferocious end to the fourth fight between the two.

Pacquiao had been down in the third round but knocked Marquez down in the fifth and the two were exchanging heavy blows in the sixth before Marquez threw a right hand that put Pacquiao face down on the canvas at 2:59 of the round.

The referee waved the fight to an end as Marquez celebrated and the sold-out crowd at the MGM erupted. Pacquiao was down for about two minutes before his handlers managed to get him up.

After being helped to his corner, Pacquiao sat on a stool, blew his nose, and stared vacantly ahead as his handlers cut his gloves off. It was a stunning end to a furious fight and it may have signaled the end of Pacquiao’s career.

‘‘I threw a perfect punch,’’ Marquez said. ‘‘I knew Manny could knock me out at any time.’’

Marquez had vowed to finally beat Pacquiao after losing two close fights and settling for a draw in the first one. But after Pacquiao knocked him down in the fifth round and was landing big left hands, it looked like it would be Pacquiao’s night.

The two came out for the sixth round and the pace was just as relentless. Both were landing big punches and both were brawling when suddenly as the round came to a close Marquez shot out a right hand that landed flush to the jaw of Pacquiao, who crumpled.

‘‘I felt he was coming to knock me out the last three rounds and I knew he was going to be wide open,’’ Marquez said.

‘‘I got hit by a punch I didn’t see,’’ Pacquiao said. Pacquiao, who earned more than $20 million for the fight, was ahead, 47-46, on all three scorecards after the fifth round.