Scott Marsden, 14, was taken to hospital after needing urgent medical attention during an English title fight in Leeds on Saturday.
Police are investigating the circumstances of his death but said they are not treating the incident as suspicious.
Facebook posts from Marsden’s All Styles Kickboxing in Sheffield, where Scott was a member, described how the fight, at Leeds Martial Arts College in Morley, was stopped on Saturday night.
The message said: “Unfortunately, (Saturday) night’s English title fight had to be stopped due to Scotty needing immediate medical attention.”
It continued: “There was no way to foresee the fight ending in such a way as at no point were either ever in any overwhelming danger from each other.
“Everyone is still in shock as there were simply no signs to suggest the fight needed to be stopped.
“The referee stepped in to protect Scott as soon as he could, which meant Scott did not hit the canvas and was laid down and straight into recovery position.”
The club said training has been cancelled until further notice and asked people to respect the family’s privacy.
World Kickboxing Association (WKA) England president Jon Green posted on Facebook: “Not just WKA but the world of kickboxing is mourning such a well-loved and respected fighter and sportsman.
“One of our finest champions in every sense in the ring and true ambassador of the values out of the ring.
“A smile that lit you up to see his face and receive it.
“In every sense it does not seem real or even possible.
“It must be said there was no failings on either teams, promoter had everything in place.”
Mr Green said ringside paramedics “fought tirelessly to keep Scott with us” but he criticised the 999 response.
“Our sport is a contact sport but every precaution is in place to have protection and to be able respond to keep our fighters safe.”
An online fund has been set up following Scott’s death.
Charlotte Rowlett, who set up the site, said: “Everything Scotty did he did with passion and a confidence that was a pleasure to behold. He was the most wonderful, talented boy and the loss of him will leave a hole in so many people’s lives.
“Scott was not only a kick boxer though, he is a son, a brother, a grandson, a nephew, a cousin and a friend.
“No amount of money can take away the pain his family are feeling today and always, but no family should ever have to cater for coping with the loss of a 14-year-old boy, with his whole life ahead of him.”
Another crowdfunding page was set up in 2015 to raise money for Scott to compete at the kickboxing World Championships in Spain.
The page said Scott had been kickboxing since he was four and entering competitions since the age of eight and that competing in Spain would be “a dream come true”.
A message on the page said: “We are a small kickboxing club in Sheffield raising funds for the experience of a lifetime for this 13-year-old kickboxing champion.
“This is a dream come true for Scott who, through his own hard work and determination, has fought his way up and qualified to represent WKA England at the 2015 WKU World Championships in El Albir, Spain.”
Tributes on Facebook said the martial arts community was “shocked and in mourning”.
Longs Black Belt School, based in North Yorkshire, described Scott as a “truly remarkable young man and martial artist”.
A member of the club said: “I am saddened to say that Scott, who was only 15 years old, died today.
“I offer my most sincere condolences and thoughts to the Marsden family. I can only imagine what you are going through.
“This is such an unbelievably sad and shocking loss to our normally very safe sport.”
The club said all the correct pre-fight medical checks and procedures were in place.
Scott is understood to have been a pupil at Forge Valley secondary school in Sheffield.
Detective Inspector Phil Jackson, of Leeds District CID, said: “At 1.20pm on Sunday, police were contacted by medical staff at Leeds General Infirmary reporting a 14-year-old boy who was in a critical condition after collapsing during an organised kickboxing contest at Alexandra Mill, Morley, on Saturday night.
“The boy has sadly since been pronounced dead.
“The incident is not being treated as suspicious and inquiries are being carried out into the circumstances for a report to the coroner.
“We are in contact with the family and are offering them every support at what is understandably a very difficult time for them.
“They have asked that the media respect their privacy at this time.”