It started as a rivalry between two distance swimmers, overnight it became a global talking point. Now it's taken a nasty and dangerous turn.

Mack Horton refused to stand on the podium alongside 400m world champion Sun Yang after being pipped by 0.73 seconds.

The Australian silver medallist made global headlines when he didn't step up alongside Sun and bronze medallist Gabriele Detti as they received their medals.

Horton's protest drew widespread criticism in Sun's homeland after he claimed the Australian had "disrespected China".

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Now the ugly feud has turned serious with Horton's Instagram page flooded with comments from upset and angry fans.

Horton previously copped heavy criticism from fans after labelling Sun a "drug cheat" ahead of the Rio Olympics, but now they've taken things too far.

Chinese fans are targeting old Horton posts with comments, mixing shots at him personally with serious threats against his family and his girlfriend.

The comments about Horton ranged from calling him a "fat Aussie pig" to a "pathetic loser" and "disgusting".

But the further you scroll the more vile the comments become with some ranging from "I'll break your tombstone tonight and dance on your grave" to "your mother's dead" and "r.i.p for your mom".

Posts of Horton and his childhood sweetheart Ella Walter also attracted death threats along with several comments of violence and those of a sexual nature.

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I love this lady 😍 @___ellawalter

A post shared by uoʇɹoɥ ʞɔɐɯ (@mackhorton) on

Sun — who served a secret three month doping ban in 2014 — faces a lifetime suspension after allegations a vial of his blood was smashed by a hammer during a clash with testers in China last year.

Reports claimed Sun objected to an out-of-competition test at his Zhejiang home last September and it was alleged his mother ordered security guards to destroy a vial of the swimmer's blood that had been taken.

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World body FINA decided not to reprimand Sun amid claims the testers had not shown adequate identification but World Anti-Doping Agency lodged an appeal.

SWIMMERS APPLAUD HORTON'S STANCE

Despite Horton standing alone behind the podium in protest, his stance was met with a standing ovation once he returned to the athletes village.

American breaststroke champion Lilly King revealed Horton was greeted with applause from fellow competitors when he returned from the pool.

"We were kind of waiting around for that awards ceremony to see what was going to happen and yeah it was awesome," two-time Olympic gold medallist King said.

"When we walked into the dining hall, he walked in after us and like the whole dining hall erupted in applause so it was pretty great to see the athletes united on his stance and supporting him as well.

"I don't think anyone at FINA's going to stand up for the athletes so the athletes have to stand up for themselves."

Australian officials spoke with FINA on Sunday night and were assured Horton did not face any sanction over his protest.

Teammate Mitch Larkin said "99 per cent" of athletes at Gwangju backed Horton's Sun snub.

"I think 100 per cent of the Aussie athletes certainly back Mack up. While he was standing alone on the podium last night he's not standing alone in spirit," he said.

"I would say 99 per cent of the athletes here support what he does. We are all fighting for a clean sport."

The rivalry between the pair in the pool will be put on ice with Horton not competing in any future events. Sun will face a Court of Arbitration for Sport hearing in September that may end his swimming career.