Callum Smith's knockout was so ferocious it left his opponent's body spasming on the canvas

Callum Smith bounced back from his loss to Saul Alvarez by scoring a knockout of the year contender, topping Lenin Castillo in London.

• Callum Smith just scored one of the knockouts of the year.
• The British fighter's finish was so ferocious it left Lenin Castillo convulsing on the canvas.
• Castillo was given medical attention, stretchered out of the ring, and taken to hospital.
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LONDON - Callum Smith delivered one of the knockouts of the year Saturday at a boxing event in Britain.

The 31-year-old wanted to rebound in a big way having been dominated by the sport's No.1 fighter Saul "Canelo" Alvarez in December, last year.

Rather than remain in the super middleweight division, and attempt to reclaim the world title he lost to Alvarez, Smith moved to light heavyweight, instead.

His first match at 175-pounds was spectacular, as he produced a second-round finish that was so ferocious it left his opponent Lenin Castillo convulsing on the canvas.

Smith had been softening Castillo up with jabs to the midsection before closing the distance to throw three pawing shots.

A devastating right hand over the top then caused considerable damage, as it buckled Castillo's legs in an instant.

Smith tried to follow up with a left hook but Castillo was already falling to the floor, where he would remain for five minutes.

The 33-year-old lost control of his body as both of his legs spasmed before stiffening.

The referee called the fight off and waved ringside physicians into the arena so that the 33-year-old could receive medical attention.

Smith's celebrated but soon muted his reaction as he realized the severity of his opponent's injury, and he instead edged tentatively around the ropes on the other side of the ring.

Castillo was eventually stretchered out of the arena and taken to a local hospital, according to the event's promoter Eddie Hearn, who is the chairman of British fight firm Matchroom.

-Eddie Hearn (@EddieHearn) September 25, 2021

The fighter is conscious and was taken to the hospital as a precaution.

It is some victory for Smith, who advances his pro boxing record to 27 wins (19 knockouts and eight decisions) against one loss.

Though Castillo was overmatched against the superior Smith, the finish will make the light heavyweight division stand up and take notice of a new entrant in the world title mix.