Swimmers at the World Championships slammed China's Sun Yang over doping allegations and said competitors were behind Australia's Mack Horton, who snubbed the Olympic star on the podium.

America's Matt Grevers said many competitors "don't trust" Sun, while world record-holder Lilly King said athletes "erupted in applause", after Horton refused to join his rival on the podium after the 400 metres freestyle medals ceremony.

However, swimming's world governing body Fina said it had decided to warn Horton over his stance.

Triple Olympic champion Sun is facing allegations, outlined in a leaked Fina doping panel report, that he smashed vials of blood with a hammer when testers visited him last year.

Fina cleared Sun to compete, but the World Anti-Doping Agency has appealed the decision at the Court of Arbitration for Sport, putting the swimmer's career on the line.

If Sun is found guilty of a doping violation, he could face a lifetime ban after serving a three-month suspension in 2014 for taking a banned stimulant he claimed was for a heart problem.

Grevers, along with King and several other swimmers, praised Horton for his "risky" protest on Sunday, after finishing second behind Sun in the 400m freestyle final in Gwangju, South Korea.

"Horton knew there were risks and felt strongly enough to still do it," Grevers said.

"It's obviously something he feels passionate about... until it gets uncovered, we don't really trust you."

Olympic breaststroke champion King revealed that swimmers at the village had applauded Horton's protest, which triggered boos from Chinese fans and prompted Sun to criticise the Aussie for being "disrespectful to China".

"Pretty baller, honestly," the American said of Horton's actions.

"It was awesome... the whole dining hall erupted in applause, so it was pretty good to see the athletes united on his stance as well.

"The athletes have to stand up for themselves, so it's definitely a start if these guys are taking a stand against doping."

Sun hit out at Horton over his podium protest, saying the Australian should have shown more respect.