The popular Mexican won his latest Cinco de Mayo bout with a staggering right hand in the middle of the eighth round, sending the raucous crowd of 73,126 into a frenzy as he waved his arms as the round continued.

Saunders' corner waved in defeat before the ninth while treating a cut under his right eye as Alvarez (56-1-2, 38 knockouts) added Saunders' WBO belt to his WBA and WBC straps.

It was the first loss in 31 fights for the 31-year-old Briton, who was taken to the hospital with a suspected broken orbital bone.

"I knew the fight was going to develop around the eighth round," Alvarez, whose only defeated was to Floyd Mayweather Jr. in 2013, said through an interpreter. "It wasn't as difficult as I expected. I know my preparation is good and I improve day-by-day."

With pandemic restrictions on attendance long gone in Texas, the festive crowd at the 80,000-seat AT&T Stadium in Arlington was the biggest in the US to date for a sporting event during the pandemic.

Organisers had expected it would break the US indoor boxing attendance record of 63,352 set in 1978 at the Superdome in New Orleans when Muhammad Ali beat Leon Spinks.

The distinction shouldn't last long with the Indianapolis 500 planning for about 135,000, or 40 per cent of the track's capacity, on May 30.

But the pro-Mexico crowd enjoyed the moment, dumping decorum by booing in the early moments of the national anthem for Great Britain and singing along to the Mexican anthem.

"It's difficult to explain," Alvarez said of his reaction to the crowd. "I don't have the words. All the emotions I feel, they give me the motivation to continue on."

After a tentative first round, the 30-year-old Alvarez took control before Saunders had his best rounds in the fifth and sixth, when the southpaw landed his best left hand of the fight. Before the eighth-round flurry, Alvarez was in control. He was ahead on all three scorecards.

In the co-main event, Elwin Soto defended his WBO junior flyweight title with a ninth-round technical knockout of Katsunari Takayama. It was the third title defence for the 24-year-old from Mexico.

Soto (19-1, 13 knockouts) almost sent Takayama (32-9) to the canvas in the first round and controlled the fight from there against the soon-to-be 38-year-old from Japan. The fight was called at 2:44 of the ninth.

Australian Associated Press