Boxer's son punches and kicks dad's opponent in the groin during weigh-in

American boxer Willie Monroe Junior wasn't prepared to be defending shots before stepping into the ring for his title tilt, let alone shots from his opponent Billy Joe Saunders' son.

At the scales prior to their WBO middleweight fight on Sunday (NZT) Monroe Jr gave what he perceived to be a playful tussle of the hair to Saunders' son on the stage, only to be met with a punch to the groin and a follow-up kick.

Saunders, the current champion, offered no apologies for his child's actions, stating he was simply putting into practice the priniciples he'd always taught him.

"My son's been taught when a stranger puts their hands on them they don't know punch and run away self-defence," Saunders wrote in a tweet.

Saunders will defend his title against Monroe at the Copper Box Arena in London on Sunday (NZT).